I know "precast" isn't in there, but it is implied and it is natural to say so in English.

Panels that will form the inside of the pool are precase in the factory. This ensures high-quality finish, etc. and high-quality placement of reinforcement (not so close to the surface that chlorine can work through and attack it, not so that it can cause cracking or spalling, etc.). Apart from reinforcing the panels (I assume reinforcement is needed), steel units will be cast into the panels so that they project on the outside. With the panels placed in the hole, with a certain gap outside them, concrete will be poured into the gap, bonding with both the back face of the panels and with the projecting steel, thereby anchoring panel to cast-in-place concrete and vice versa.

Just as I think you can add "precast", I think you can omit "actif". It is implied in "permanent", and simply means that the panel forms a part of the final whole, is not formwork that is subsequently removed (permanent).

That said, there is a slight possibility that it might be deemed to play an active role in withstanding water and ground forces, i.e. the concrete behind it would not be thick/strong enough by itself, but relies of forming a monolithic whole with the panel. But I suspect not. I think they think it just sounds snazzier.