NFL free agency is now just a month away, and as we begin the process of previewing the 2018 free agent class, we'll start with a less prominent part that is still relevant for the Bucs this year: restricted free agents.

Unrestricted free agents have four or more accrued NFL seasons and can sign wherever they want on the open market. Restricted free agents, by comparison, have only three accrued seasons, and their original team begins the process by "tendering" them with a one-year contract that determines just how "free" they are as free agents.

Since all draft picks sign four-year contracts — first-rounders have fifth-year options on top of that — restricted free agents are either players who weren't drafted at all, or were drafted and later cut and cleared waivers.

The Bucs have five RFAs — TE Cameron Brate, WR Adam Humphries, LB Adarius Glanton, DE Ryan Russell and DB Jude Adjei-Barimah — and they serve as good examples of the decisions teams must make in issuing an RFA tender.

If a team issues a "first-round" or "second-round" tender, not only can they match any offer that comes from another team, that other team would also have to give up a first- or second-round pick to sign away the player. Those tenders are expensive — last year, it was $3.91-million for a first-round tender, $2.74-million for a second.

A team can issue a lower tender — last year it was $1.79-million — that allows them to match any outside offer sheet for an undrafted player, or get back a draft pick equal to the original round the player was drafted in. For most players, this is still almost triple the league-minimum salary they made the previous year.

This brings us to the Bucs RFAs, and decisions on how they'll be tendered in the next month. If an RFA isn't tendered at all — if the team doesn't think they're worth that lowest tender — they become unrestricted free agents and can hit the open market to go anywhere.

CAMERON BRATE, tight end: Brate, 26, has been remarkably productive, especially as a red-zone target for Jameis Winston, with 14 touchdown receptions in the last two seasons.

That he's only a restricted free agent is due to NFL fine print — to have an accrued season toward free agency, a player must be on roster for six weeks, and the Bucs promoted Brate for the last five games in 2014, so while he's played in four NFL seasons, he only gets credit for three.

Anything less than a second-round tender for Brate would leave the Bucs vulnerable to another team making an expensive offer and Tampa Bay either matching at a high price or declining and getting nothing in return.

And since Brate has played four years and has only earned about $2-million total, he might agree to a longterm deal now, to get some guaranteed money and guard against an injury in the upcoming season. Spotrac.com puts Brate's open-market value at $7.9-million per season, so the Bucs could offer him an extension at less than that now to get big money sooner.

A conservative comp would be the Colts' Jack Doyle, who got the low tender as an RFA in 2016, then signed a three-year, $19-million deal to stay with Indianapolis last spring. Brate has done more and will command more than that. There aren't a ton of good tight ends available this spring — Jimmy Graham would be the high end, with Austin Seferian-Jenkins and Trey Burton among the younger upside types like Brate. (Burton had only one career TD before this season, but could be in for a nice payday now.)

A team can also trade a restricted free agent — when the Bucs made an offer to DE George Johnson in 2015, the Lions leveraged the threat of matching and got a fifth-round draft pick from the Bucs. So with a high tender, the Bucs could still in theory deal Brate for a mid-round pick, knowing they have O.J. Howard and will eventually have to pay him at a high level.

Tampa Bay Buccaneers wide receiver Adam Humphries (10) stretches after a catch in the fourth quarter of a game vs. the New York Giants. (Times file, 2017)

ADAM HUMPHRIES, receiver: Like Brate, Humphries has been wildly productive as an undrafted player. Amid injuries, he stepped up in the slot with 55 catches for 622 yards in 2016, and last year, with no major injuries, he improved with 61 catches for 631 yards. He's only 24, and the Bucs are already paying Mike Evans and DeSean Jackson a combined $24-million in 2018, with Chris Godwin emerging as a playmaker as well.

Humphries is sure to be tendered, and the popular speculation for an outside suitor is the Patriots, who are as active with RFA offer sheets as any NFL team. Two years ago, receiver Chris Hogan had more modest stats than Humphries (87 career catches, 759 yards, 6 TDs with Bills) and he got a three-year, $12-million deal from New England. They got RB Mike Gillislee with an offer sheet last year, and tried to get WR Emmanuel Sanders a few years back but the Steelers matched after under-tendering him.

New England has Danny Amendola at 32 and Julian Edelman at 31, and Brandin Cooks will make $8.4-million in 2018 with free agency ahead next year. They did give up QB Jacoby Brissett to get Phillip Dorsett, who had just 12 catches for 194 yards and no touchdowns in 2017.

Even with a low tender, Humphries would be rewarded with more money than he's made in his first three NFL seasons combined.

Tampa Bay Bucs outside linebacker Adarius Glanton (53) takes the field before a game vs. Dallas. (Times file, 2017)

ADARIUS GLANTON, linebacker: The Bucs have three solid linebackers ahead of Glanton, so his basic role is as a special-teams player. But when starters have gone down with injury, he's played well, getting 20 tackles in a span of three games last season when pressed into a larger role.

Glanton, 27, made just $690,000 last year as a relative steal — a low tender would nearly triple his salary for 2018, but would shore up depth at linebacker against future injuries. One complication is that Glanton broke his lower leg in the 2017 season finale, but is expected to make a full recovery by the start of training camp.

RYAN RUSSELL, defensive end: With all the injuries at end, the 26-year-old Russell started seven games last year. Two sacks isn't anything impressive, but that was a half-sack off the team lead among defensive ends. Having invested a year and a half in Russell, do the Bucs tender him to keep him around to contend for a backup pass-rusher spot in 2018?

Bucs Jude Adjei-Barimah (38) sacks the Raiders’ Derek Carr (4) in overtime of an Oct. 30, 2016, game. (Jim Damaske, Times)

JUDE ADJEI-BARIMAH, defensive back: It's been a while since Adjei-Barimah has played — he missed all of 2017 with a knee injury, and missed the final six games of 2016 between a four-game suspension and injury.

He started seven games in a bad Bucs secondary in 2015 and has flexibility to play outside and at nickel, so do the Bucs tender him to help their depth in what's sure to be a revamped secondary in 2018? Do they gamble that coming off an injury, they can bring him back for less than the tender, knowing he'll prefer the familiarity of the Bucs defense to a new situation?

EXCLUSIVE-RIGHTS FREE AGENTS: This is something of a misnomer —players with only two accrued seasons are ERFAs, and all a team must do is offer them a one-year, league-minimum $630,000 contract (with no guarantee) to keep them in the fold.

For the Bucs, that's just RB Peyton Barber and DB Javien Elliott, and you'd think both have earned the chance to fight for roster spots in camp this fall. Barber led the Bucs in rushing so he's almost certainly back with new competition around him. Elliott only had five tackles all season despite all the injuries at corner, so he's probably on the outside in again, battling for a backup spot on the roster.