Coil and contact configuration
The SD-T21BCDC24V 2A2B carries a 24 VDC coil — the DC coil draws steady current without the inrush spike of an AC coil, which simplifies the control transformer sizing and eliminates the hum from the magnetic circuit. The 2A2B suffix means two normally-open (2A) and two normally-closed (2B) auxiliary contacts built into the contactor body. That gives you four auxiliary contact positions without an add-on side-mount block — useful for feedback to a PLC or for interlocking a reversing starter.
The lifecycle stage is current, meaning Mitsubishi Electric still lists this part as an active catalog number with no announced phase-out.