tax abatements

hi, there has been a couple discussions being thrown around on 421a abatements but i didnt really see anything definitive given that most if not every new construction in nyc has one. i have a two questions.. 1) are the 421 abatements only effective upon getting a permanent c/o, ie, if you close into a TCO, are you paying the unabated tax until the perm c/o is approved? dont have a contract out there, so i dont have a lawyer to consult.. but as someone has said previously, this is truly one of the shadiest parts of the process.. i imagine more and more new devs will be conducting closings on TCO as sales slows and developers increasingly gets squeezed on liquidity.. thanks in advance!