A short kit contains the parts that need to be cut to a specific shape, such as wing ribs, fuselage sides (especially wing saddles), bulkheads, and other tidbits that would cause the plane to be out of whack if not cut correctly but are tedious to cut out with a hobby knife. Here’s a photo of my short kit of the Sporty Forty biplane, taken by the guy who bought it from me. It is notable that this kit includes the full tail of the plane, including vertical and horizontal stabilizer, elevators, and rudder. This is not true of all kits on my list. Some have tails built of sticks.
The kit alleviates a lot of meticulous cutting, but it does not alleviate buying spars, sticks, triangle stock, sheet wood for the fuselage bottom and wings, landing gear, wheels, hardware, engine mount, and any other standard stock that might be required to finish the airplane. Here’s my idea of how to make a list of materials needed to complete the plane.
A scratch builder has to collect all of the parts necessary for completion from diverse sources. When you buy a short kit you are a scratch builder who pays somebody to cut out the fiddly bits.