Man... I think we could write two or three books on what rubs mean... And I would still probably be wondering if we were right on all of the info.
I would say a buck might rub a tree any time any where he might be for any reason he feels like...
But, what we are really looking for is the types of rubbing activity that can help us as a hunter. 1st off, I truly believe the oldest biggest bucks, don't rub much. Most of the rubbing occurs from 2 1/2 to 3 1/2 year old bucks.
One of my favorite types of rubs is where a buck rubs the cross trail of a doe.
The reason I like these types of rubs is they are made during daylight most of the time and revisited a lot ( during the rut )
The places I find these cross trail rubs are where a buck skirts around the perimeter of a doe bedding area smelling each trail where does head to the bedding area. He will rub the trees right where the trails intersect. Some people will mistake these rubs thinking they are on the doe trail cause the buck trail is often faint.
You can also find these rubs where a buck will cruise 40 or so yards into a woods along a field and check all the doe trails heading into or out of the fields to feed / bed.
Another huntable rub is the classic rubline that comes out of a bedding area. But in my opinion, they are usually made by bucks that have not reached maturity yet...