What is a flexible waterproof casing through the wall?

Casing is generally used when making a pool, and it is a channel for pipes leading to the pool. But ordinary casing is difficult to seal, so there will be leakage between the casing and the pipe, and between the casing and the wall.
 
If you only use the waterproof casing, you can solve the leakage between the casing and the wall, but you can't solve the sealing between the casing and the pipe.　
 
Flexible waterproof casing can be divided into: ordinary casing: 2-4cm thicker than the pipe to be used, fixed in the wall or plate, from which the pipe passes. Flexible casing is the sealing effect between the casing and the pipe with flexible material.
 
Steel casing is the sealing effect between the casing and the pipe by sealing with steel material. Waterproof casing is the casing used to prevent liquid leakage or leakage into the environment (such as pools, basements, etc.), is to add no less than one circle of waterproof wings on the outer wall of the casing, so that when the casing is poured in the wall body becomes a whole, will not be due to expansion and contraction factors and cracks and leakage.