In order to standardize all responses, let's base severity on a 1995 vehicle that's been driven all year round in any weather that is stored in a garage.

Spotless:

Nothing rusts. A 35 year old car's underbody is going to look like it did from the factory (but much dirtier).

Minimal:

Some of the "replaceable" parts may have very light surface rust on them such as suspension, brake drums, exhaust, nuts and bolts, but definitely no frame or body rust and certainly no rot. Probably don't need WD-40 to get any bolts off.

Light:

"Replaceable" parts probably have some surface rust on them such as suspension, brakes, etc. May have some light rust on the floor pans or on the frame but rot/holes is not common. Rockers don't have holes, fenders don't have holes. Brake lines and fuel lines don't rust often if ever. Bolts still generally come off easy with a little WD-40.

Moderate:

Brake lines and fuel lines rust as do the replaceable parts such as brakes, suspension, nuts and bolts, etc. Floor pans and frame can also rust. Fenders and rockers probably have some holes in them and the "high spray" areas may show the heaviest signs of rust. You can probably get by with PB blaster but a torch is nice.

Heavy:

Floor holes are certainly the norm on 15+ year old cars. Frame probably has rot, bolts are stuck on often, rockers have holes, bottom of doors, fenders, other body panels probably have some rust through. Doing most any job sucks and will require heat.

Severe:

A torch is a must have tool in your garage and the book time is completely irrelevant. It's often easier to cut it off than to try to unbolt it. Your 15+ year old car might have no floors and allows you to drive it "Fred Flinstone" style. Frame probably has some rot on it, bottom of doors, trunk, etc have rust holes and bubbles. Rocker panels are at least half gone.