Radiators and coolant recovery tanks. It's actually quite simple HeavyOp, when things are hot they expand, when they're cold, they contract, this includes engine coolant. In the old days, radiators had large reservoirs on them, generally on the top of radiator. One would fill these radiators to no more than 1" from filler neck in top of radiator with engine/coolant cold. When engine/coolant warmed up, the coolant would expand and take up that 1" of air space in top of radiator reservoir, no coolant recovery bottle was used or needed.
Coolant recovery bottles came to use in automotive when the nose of cars got smaller, no room for bulky radiators with large tanks. This also when they moved the radiator reservoir from top of radiator to sides, again, to make radiator less tall. These more compact radiators left no extra room for an air space for expansion of coolant when warm, therefore, recovery bottle was incorporated. With this type of cooling system, radiator is filled completely with coolant with engine cold, recovery type radiator cap is fitted, then you fill the recovery bottle to the "COLD" mark on recovery bottle. As engine/coolant warms up, coolant expands, is forced through recovery cap, and goes to recovery bottle. When engine is shut down and cools off, coolant in system contracts and draws coolant back into system from recovery bottle.