A change in FINA rules will now require all relay alternates at the 2016 Rio Olympic Games to compete in either prelims or finals.

Traditionally, nations with enough swimmers under the FINA “B” cut will bring up to 6 swimmers per relay, giving them the option of resting some swimmers in prelims, or letting a few swimmers compete in prelims for the right to a slot in the finals relay.

But FINA’s qualification system for the 2016 Rio Games has a new section that complicates that strategy somewhat:

If an NOC [National Olympic Committee] enters relay-only swimmers for a specific event, these swimmers must swim either in the heat or final of that relay event. Should a relay-only swimmer not compete, this will lead to the disqualification of the respective team in the final.

That section is not in the qualifying procedures from the 2012 London Olympics, which you can read here.

The biggest impact of the new rule will be that federations will have to make their decisions early on as to who will swim prelims and finals of the freestyle relays, because those decisions will affect how many relay-only alternates to bring along to Rio.

Of particular note is the U.S. men’s 4×100 free relay, which missed the final at the World Championships last summer. USA Swimming will have to weigh out its options, deciding whether a relay team with two alternates can still earn a lane in the finals, and if that risk is worth resting a few swimmers in prelims. Michael Phelps and Ryan Lochte in particular have previously been members of that relay who have taken on difficult individual event lineups that made it an attractive option to swim alternates in their places in prelims.

On the other hand, if the London Olympics are any indicator, the U.S. men should have no qualms with including and swimming multiple alternates in Rio. The London 4×100 free relay swam an entirely different lineup from prelims to finals, and still took second in both. The 4×200 free relay only had Conor Dwyer compete in both prelims and finals, swapping out the other three legs. That team was first out of heats and won the gold in finals.

USA Swimming has not released its official selection criteria for the U.S. Olympic Team yet, but it will be worth keeping an eye on the criteria when they do, as the nation might consider leaving 5th- and 6th-place finishers in the 100 and 200 freestyles up to the federation’s discretion to be added or passed over as Olympians depending on each relay’s relative strength compared to the rest of the world.