Looking for some info/confirmation on using the <any> element.

I have an AbstractBaseObject with properties id, createdOn, createdBy, updatedOn, updatedBy. Most all of my domain objects extend this. There's no table for the Abstract class and each of the concrete classes has its own table.

When exactly is the <any> element needed? Only if I plan on returning all subclass instances? In other words, if I didn't need this, is <any> required? I'm thinking no.

I read the bit on <any> in the documentation for class 'payment' but am a little unclear as to where it goes/what and when it's needed? Could someone explain or point me to some code examples using it?

Thanks,
Mike