The 387600318: The 0.375" (9.53 mm) pitch sets the screw-terminal centers — wide enough for 14 AWG ring terminals without crowding, but the footprint on the panel or chassis needs an opening that matches the 18-circuit length. The UL94 V-0 rating means the housing will not sustain a flame after ignition — required for equipment inside a plenum or enclosure.
The 387600318 (18 circuits) shares the same 0.375" pitch, 15 A, 300 V, flange-mount, and dual-row layout as the 387600310 (10 circuits). If the BOM calls for 18 terminations in a single block, the 387600318 is the match; the 10-circuit 387600310 will not fit the same panel cutout or provide enough positions.