Once energized, a stick relay uses a front contact, one which closes when the relay picks up, to keep itself energized. Some other condition in the associated circuitry has to occur to de-energize it. They can be neutral relays, polar relays, or biased neutral relays. In fact any relay, not just signal relays, can be wired to stick itself up. A stick relay isn't per se a mechanical latching relay.

It's possible to use diodes for the same purposes (traffic direction detection and control) but the reason GRS used stick relays in AP-B is that 1918 predated the development of usable rectifiers.