What are the rules that CP11 uses to determine whether a domain should added to etc/secondarymx when DNS changes are made?

I assume that it would do this if it saw -
1. More than MX record exists for a domain and
2. The lowest MX priority is a remote address and
3. One of the other MX records is a local address.

I don't have root access to the server I'm using at the moment, so I can't see what WHM is doing to etc/secondarymx when I make DNS changes which makes it hard to problem solve.