What's this button on Calendar.app?

On OS X 10.9.5, the Calendar.app toolbar sometimes adds this button:

There's no tooltip, and the Calendar toolbar isn't editable so there's no way to show a label for it. I can't find anything in the online documentation about it.

I think it has something to do with remote/subscribed calendars, but what, exactly?

This is the Notifications button.

The Notifications button shows how many invitations you have. If you don’t have any invitations, it isn’t available.

^{Source: Calendar: Reply to an event or calendar invitation}

The notification button also shows when changes are made to a subscribed calendar. In this case, the standard Accept/Decline/Maybe buttons are replaced by a single OK option to dismiss.