This is about the Unix Select operation.
A program may need to wait for the next response to any of several external activities that it has instigated. Normally separate activities should be made by separate domains each with holding the context for that activity, but sometimes these contexts are necessarily entangled and one program state is the convenient way to handle these responses.
Perhaps this is the best excuse for the Unix Select kernel operation. In the case of Unix these activities are file operations which Unix has expanded to include many sorts of external activity.
In Keykos instigation is normally done by invoking a start key