This MUF syntax allows to filter by child table. A typical use is for group of users:
Logically, one wants to filter the users - but what if we don't know all the users the MUF should filter to, only their groups? In that case, OVER-TO comes to play - we can use the "GroupUser" table (also usually called a "membership" table) to filter over. When used, the project will act like there is an MUF on the USER table, allowing for all users that happens to be members of the groups that the filter actually allows. See GoodData writeup attached.