Texture streaming in open world game: which memory is using and what values are suggested

Hello, I would like to understand better the ue4 texture streaming system.
I read the documentation but I can’t find informations on which memory texture streaming uses.
RAM? Video memory?
Which is the value I should set? The default one is 400 mb but our game is an open world and it goes easily over 500 mb after 2-3 minutes of gameplay.
Now I set the texture streaming pool to 1000 mb and everything is ok, but I don’t know what this implicates.
A large texture compression and study for memory optimization has been done, so I don’t have 2k textures, just 1k and 512, no roughness on most of the objects and 512 normal everywhere.
Could someone explain it better please? I noticed using list streaming command that there are a lot of landscape texture loaded several times…
Thank you very much!

No one can answer me?