The Navy SEAL was an infantry unit cut from Generals.
Essentially being the USA's analogous equivalent to the Red Alert 2 SEAL, American SEALs would have likely performed the same role as Burton does in the final game; a skirmish unit excelling at anti-infantry and base sabotage.
However, much of the SEALs supposed roles overlap with other infantry units; anti-infantry by the Pathfinder and base sabotage by Burton. Therefore the SEAL was removed before it could be coded into the game.
Only the incomplete model and an unused cameo remains within the game files.
Several non-canon fan modifications allow players to train the Navy SEAL, more or less performing the same roles SEALs were intended to be able to perform originally. The most notable example being the Zero Hour modification Rise of the Reds.
In addition, SEALs are usable by several US factions in the Shockwave mod though they go under the designation of Marines.