The developers behind the ginormous tower complex planned for on and around South Station have made official at least one thing we’ve known for a while: The tallest part of the complex, a 43-story addition to South Station, will include 175 condos on the upper floors.

The tower is slated to rise to 677 feet in total and to 640 feet at the highest occupiable floor. The former figure all but guarantees that the spire will become one of Boston’s 10 tallest buildings by 2020.

Incidentally, the tower will run to 51 stories altogether, with the addition starting at the ninth floor with what the developers have dubbed a "Sky Lobby."

Those developers, Hines and Gemdale Properties, have also elaborated on the other two parts of the complex in recent filings with the city and the state:

A 17-story, approximately 438,000-square-foot building will be either a 360-room hotel; a residential building (of either 260 condos or 375 apartments); or a mix of both uses, plus retail. It depends on the market as it nears completion.

A nine-story building of approximately 511,000 square feet is slated to included offices and retail only.

Here is the developers’ filing with the Boston Redevelopment Authority (PDF). Scroll to page 14 for the start of the descriptions of the three buildings.

As for construction, it’s not expected to start until 2017, pending the necessary approvals. So stay tuned.