They crack along the seam and at the hose nipple, which is why a vehicle can lose coolant with no visible puddle under the engine.
Listings state whether the tank includes the pressure cap and the level sensor. On many applications the cap is a separate pressure-rated part and the sensor is a separate float, and buying a bare tank means reusing both.
A tank that keeps losing coolant after replacement usually has a failed cap or a head gasket fault pressurising the system. Check the cap first, then look for combustion gases in the coolant.
Tanks are also listed for brake master cylinder reservoirs, which are a different part on a different system and are found under brake hydraulics.
