What is the purpose of the "member release" command? What is the basis for the terms MX, MY and MZ in this command?
By default, STAAD assumes the connection between any 2 members to be fully capable of transmitting all 3 forces and all 3 moments from one member to the other. This is usually achieved in practice by moment resistant connections, such as between a concrete beam and a concrete column which are monolithically cast.
If you want the connection to be of the type which does not permit one or more forces/moments to be transmitted, use member releases. A shear connection is such an example. The degrees of freedom FX through MZ that you release are based on the local axis of the member at whose end the release is specified.
See section Technical Reference of STAAD Commands > TR.22 Member and Element Releases > TR.22.1 Member Release Specification in STAAD.Pro Help for additional information.