This functionality is available on Onshape's browser and iOS platforms only.

Automatically create a Mate when inserting an entity into an Assembly, using explicitly created Mate connectors on the entity being inserted. Also, drag one entity (by a Mate connector) to snap to a Mate connector on another entity. Both methods result in an open Mate dialog so you are able to fine tune the Mate type and alignment. Snap mode is able to be toggled on or off.