Now what do we exactly mean by partial drain.
On nearly all vehicles approximately
1/2 of the coolant is in the radiator/overflow tank and the other 1/2 is in the engine block. Most here refer to a partial drain as draining and replacing the 1/2 that is in the radiator/overflow tank only. Because today's coolants keep the systems so clean, it is felt unnecessary to do an entire coolant flush. It's similar to just doing a pan drop and refill on a transmission, where 1/2 (+/-) of the old fluid remains in the torque converter, etc..
The logic of the partial drain done more often is that it is just so easy to drain and refill the radiator compared to attempting to flush out the entire engine. And again, we just don't seem to see the crud buildup in cooling systems with the newer coolants like we did several decades ago. Save the complete fluid exchange for neglected systems or when changing coolant types.