With supervisory control theory it is possible to describe controllers which influence the behaviour of a system by disabling controllable events. But sometimes it is desirable to have a controller which not only disables controllable events but also chooses one among the enabled ones. This event can be interpreted as a command given to the plant. This idea is formalized in the concept of an implementation, which is a special supervisor, enabling at most one controllable event at a time. In this paper, some useful properties are introduced, which ensure, when met, that each implementation of a given DES is nonblocking. The approach is applied to a simple chemical batch process example.