Neither, according to FCR's latest formulation. A reader directed me to another dubious statement on AtlanticYards.com web site:
Located at the intersection of Atlantic and Flatbush Avenues, bounded by Pacific and Dean Streets and Vanderbilt Avenue, and primarily situated over the MTA/LIRR’s Vanderbilt rail yards.
The formulation is repeated:
The Atlantic Yards development will be built primarily over the Long Island Rail Road's Vanderbilt Rail Yards.
The dictionary defines primarily:
1 : for the most part : CHIEFLY
2 : in the first place : ORIGINALLY
Because the railyards would be 8.3 acres out of a 22-acre project, you couldn't say that the first definition applies. And given that the railyards are located between Pacific Street and Atlantic Avenue, and the developer needs the blocks between Dean and Pacific streets to build the arena and the first cluster of towers, it would be tough to say that the second definition applies.
Yes, this formulation is less of a fib than the claim that the project would be built over the railyards. A more accurate description would be that the railyards represent a plurality of the project footprint, meaning a larger area than any other plot of land but still less than half. But there's no adverbial form in the dictionary and it would be a tongue-twister anyway.