Ok. I think I get what you're saying.
That property is NOT meant to override a classic domino URL It's an XPages only property so you need to be in the XPages world.
Typically its used in stuff like a view control. You drop a view control on an xpage and set a column to be a link to open the document. The view control has a setting of what form to use BUT by default it will look at the documents form to find the XPage to open.
So basically any XPage link that opens a document will by default look at this property on the form to see what XPage to use for the document.