Pacemaker allows you to tag any configuration element that has an XML ID.
The main purpose of tagging is to support higher-level user interface tools; Pacemaker itself only uses tags within constraints. Therefore, what you can do with tags mostly depends on the tools you use.
What you can do with this new tag depends on what your higher-level tools support. For example, a tool might allow you to enable or disable all of the tagged resources at once, or show the status of just the tagged resources.
A single configuration element can be listed in any number of tags.