It's rated for a maximum of 10 A and carries a 690 V insulation voltage with degree of pollution 3, so it's built for industrial control panels where contactors are switching motor loads. The screw-type terminals accept 2x (0.5 to 1.5 mm²) or 2x (0.75 to 2.5 mm²) with core-end processing, or the AWG equivalents 2x (20 to 16) and 2x (18 to 14).
Snap-on mounting onto the contactor means no tools needed for installation — just push it onto the 3RT1 base unit until it latches. Dimensions are 44 mm wide, 38 mm tall, and 51 mm deep, which keeps the assembly compact alongside the contactor.
The 3RH1921-1MA20 carries 2 NC contacts only. The successor 3RH2122-1WB40 carries 2 NO and 2 NC, so if your circuit needs both normally-open and normally-closed auxiliary contacts, the replacement block gives you that flexibility. The 3RH2 blocks are also slightly larger — 45 mm wide and 73 mm deep versus the 3RH1921's 44 mm by 51 mm — so verify panel clearance before swapping.