As of now it doesn't seem like this variable is being used.
Anyway the purpose of this variable is to determinate how much containers Appwrite is allowed to keep for functions.
For example if you have 10 functions, then no matter how many execution each function will have, Appwrite won't create more then 10 containers, as each function gets one.
But if let's say you had 20 function and the value set to 10 then when the 11th function will trigger Appwrite will shutdown one of the other containers,
Also, it's limit only by our imagination and server resources.
But, from what I see it's not being used so it's all just theory