There're two checkboxes - Supports re-Invite, and Supports 'Replaces'.
First tells PBX if it is allowed to renegotiate codecs for that endpoint. If not - it will bound it to media server as a call starts.
Second can be used only with re-invites, so it is grayed out if re-invites are disbled. It is just optimization hint for attended transfer. You should untick it if you have problems with attended transfers that involve that endpoint.