Three keywords are used to specify whether a channel is served by a master program (master), a slave program (slave), or both (bidirectional). The default, if none of these keywords are specified, is bidirectional. These keywords determine whether IMTA initiates delivery activity when a message is queued to the channel.
The use of these keywords reflect certain fundamental characteristics of the corresponding channel program or programs. The descriptions of the various channels IMTA supports indicate when and where these keywords should be used.