The GROUP clause, which need not be present, is repeatedly used to name each object and notification group which is conditionally mandatory or unconditionally optional for compliance to the MIB module. A group named in a GROUP clause must be absent from the correspondent MANDATORY-GROUPS clause.
Conditionally mandatory groups include those which are mandatory only if a particular protocol is implemented, or only if another group is implemented. A GROUP clause's DESCRIPTION specifies the conditions under which the group is conditionally mandatory.
A group which is named in neither a MANDATORY-GROUPS clause nor a GROUP clause, is unconditionally optional for compliance to the MIB module.