The Many-To-Many selector is a form control that allows you to manage Many-To-Many relationships. You can save the relationsships to a Join Table, a Custom Module Table, or save it as a delimited list.

See the Guide attached in the download package for more information.  Also see my blog article on this tool, however note that the blog article outlines customization that is no longer needed in the current verions.

This is included in the RelationshipsExtended module, so if you get that you don't need this.