28 A motor rating — what drives the selection
The A26-30-10-84 is a 3-pole IEC contactor rated 28 A in AC-3 duty for motor loads and 40 A for general-purpose resistive loads at 600 VAC. The 28 A figure is the one you size against the motor full-load amps — the 40 A general-purpose number covers heater banks or resistive loads where the inrush is lower. For a 10 HP motor at 460 V (roughly 14 A FLA), this contactor gives 2:1 headroom; for a 20 HP motor at 460 V (~27 A FLA), you're at the edge and should step up a frame.
Coil voltage and auxiliary contact
The 120 VAC coil is standard for North American control transformers — the coil draw is under 10 VA sealed, so a 50 VA control transformer can drive several of these plus a PLC output relay. The built-in 1 N.O. auxiliary contact saves a side-mount block for feedback to the PLC or a seal-in circuit. On a motor starter with a separate overload relay, that N.O. contact handles the interlock wiring without eating a spare contact on the relay.