What the 120 mA / 350 V rating means for your BOM
The AQY210HLA is a PhotoMOS™ solid-state relay from Panasonic's AQY series, switching AC or DC loads up to 120 mA at voltages from 0 to 350 V. The 25 Ohm on-state resistance at that current means the voltage drop across the output is about 3 V at full load — negligible for signal switching but worth budgeting if you are driving a long cable or a low-impedance load.
Surface-mount in a 4-SMD (0.300", 7.62 mm) package with gull-wing leads — the footprint is the same as a standard DIP-4 optocoupler or small signal relay, so it drops into existing layouts without a respin. The 4-SMD supplier device package is the same body as the through-hole version but with surface-mount terminations.