It is 10 SEER, not 10t. SEER means seasonal energy efficiency ratio.
The higher the SEER, the cheaper it is to operate it. If you are going
to use it more than 3 to 5 years, a 12 to 13 SEER will be cheaper to
own. Also make sure your contractor does a load calculation instead of
using the 500 Sq. Ft. per ton rule of thumb. It will be more
efficient, last longer and dehumidify better than an oversized unit.