Affiliations are User Groups that volunteers are able to self-select into.  On the Admin-side they are User Groups with the membership control set to 'Checkbox'.

Utilizing 'Checkbox' User Groups

Volunteers initially encounter the Affiliations page when creating their account as the step before filling out their profile (User Form):

Volunteers can also modify their Affiliations after account creation by clicking on their name at the top of Volunteer View and choosing "Manage Affiliations":

Some organizations may choose to update their checkbox groups, perhaps even seasonally.  Doing this might require all volunteers to review and update their Affiliations.  When any user accesses the Manage Affiliations page the URL is always the same, it is simply who is logged in that changes.  Because of this, an administrator could email that link to all their volunteers with instructions to access the page and update their selections.  Likewise, a link or button to that page could be placed within the Schedule Message to more quickly direct users to update their memberships.

The Manage Affiliations page is always: