AUM (assets under management) is a core part of the private banking world and is often a prerequisite to securing the best possible terms.
Some private banks require a minimum amount to be invested with them; some require a percentage of the new private mortgage to be placed with them either as custody or to manage. A few private banks operate on the potential of a new client to bring assets and of course, there are plenty of private banks who are happy to lend without needing anything in return.
As with everything else, the amount of AUM needed is often fully negotiable and can be structured in lots of different ways.