B12 is supposed to be a condo building, while B15 would contain market-rate rentals, plus a school. Both have 421-a benefits grandfathered in, but the developer could reconfigure the building to use the revised benefits.
The evidence is below. A search on "Pacific Park" turned up four limited liability corporations (LLCs) associated with specific parcels, including the now-constructed 550 Vanderbilt and 535 Carlton, and the under-construction 38 Sixth Avenue.
The only other construction site with an LLC is 615 Dean. (Note that 461 Dean, aka B2, was built not by the joint venture behind Pacific Park, Greenland Forest City Partners, but by Forest City itself.)