MCX312A-XCBT can be used with both SR and LR modules - for example with these Mellanox modules (also qualified with similar modules from other major vendors):
MFM1T02A-LR Mellanox optical module, ETH 10GbE, 10Gb/s, SFP+, LC-LC, 1310nm, LR up to 10km
MFM1T02A-SR Mellanox optical module, ETH 10GbE, 10Gb/s, SFP+, LC-LC, 850nm, SR up to 300m
BTW, they also can be used with 1GE modules:
MC3208011-SX Mellanox® Optical module, ETH 1GbE, 1Gb/s, SFP, LC-LC, SX 850nm, up to 500m
MC3208411-T Mellanox® module, ETH 1GbE, 1Gb/s, SFP, Base-T, up to 100m