I've only been a member of this forum for a few months, but there seem to be about 5 categories most of the bikes around here could fit into.
Hot rod
Rat rod
Custom
Freak bike/True Rat
Original or mostly original
Hot rods are generally old bikes with glossy paint, often in non original colors. Often lots of chrome and lots of style. Copious upgrades in the performance/style department are common.
Basically a rat rod to me is a hot rod without the fancy paint and fewer or more subtle upgrades. Anything other than original (and well worn) paint, rust, primer or flat paint is out. A good rat should have a few tasteful performance/style oriented upgrades, but nothing over the top. Good things to upgrade might include wheels, tires, cranks/chainring/pedals, and seats. Period correct parts is a bonus but where performance is concerned, modern stuff is fine. Hand made stuff is really cool too. Rusty stuff is fine, no need for shiny chrome on these bikes. Basically anything goes other than fancy paint, but subtlety is good.
Newer styled bikes with some wicked modifications are customs, but not rat rods or hot rods. They may be modified to look older or be faster. They generally have nice paint.
Pure rats and freak bikes are rusty, junky, kludged together beasts that may or may not be easy or comfortable to ride. Beater bikes built with parts that dint seem to go together along with tall bikes, choppers, trikes, cargo bikes fall into this category. I haven't seen many bikes in this category here, but I think most forum members can appreciate these bikes more than your average bike enthusiast.
Original bikes are older bikes that have all or most of the original parts in various states. Parts that are not original should be period correct. These bikes should be fully functional and easy on the eyes so as not to get lumped in with the rats and freak bikes.
Though not truly original, I would lump a restored bike in with these.
