Hi, I was wondering about when to use pools.

For context, I'm building my first plugin wich is called Career and removes the techtree and research tables to replace them with an xp based system where you chose between differrent careers to unlock BP's and skills (mechanician, soldier, medic, farmer, etc...).

So i Have a UsersManager class instanciated once when server is initialised, with 1) a UserDatas struc in it (with the levels, permissiion, etc) and 2) 2 dictionaries <id, UserDatas> : one for active users and the other for stored users. (+ 3) methods to save, update, etc)

I have a GUI builder class being instanciated with a BasePlayer parameter menu everytime a chat command calls for menu.

And I have an XP manager static class granting xp on hook calls and calculating levels etc for every user (who has tempXP) on a timer.

Should my Dict of users be in pools ? I think not as they are created once and are meant to be always available.
Should my GUI builder class be in a pool ? I think yes, as it is instanciated like a "throwable" for unique usage and short time period.  
Should my XP manager class be in a pool ? I think not as it is static

Could someone expand a bit on pool usage and/or on my particular case ? I'm not sure to fully understand how they work and what they do. Thanks.