It is a set of connections — the mechanical bridge and wiring links that turn a pair of contactors into a reversing starter. Rated 25 A operational current, it mates with LC1E18 through LC1E25 frames.
What the 25 A rating means for fit
The 25 A figure is the rated operational current (Ie) for the connection set under normal load. That matches the contactor frame it fits — LC1E18 through LC1E25 — so if your reversing starter is built around those contactors, this connection set is the correct mechanical interlock and power wiring bridge. It handles the full motor current the contactors are rated for, up to 25 A.