UI Widgets - Invalidation Box and Retainer Box for Mobile Android Game


I have read a few documents online and the Unreal Documentation but still don’t really understand when I should use Invalidation Box or Retainer Box.
I have read some people say to use Invalidation box on **ALL **and EVERY UI Widgets.
Others online have said if the UI widget keeps changing (size, shape, Numbers) do not use them and to use Retainer Box instead.

Invalidation Box - cache Slate Tick data without calculating every frame and not Prepassed, Ticked or Painted. Retainer Box - can control frequency and phase of the UI Rendered when UI does not need to be rendered every frame.

So, my question is when should I use them. For example, My UI Widget for my Mobile Android Game will contain:

Player Name and Image: which Will remain the same throughout the level ----- Invalidation Box?
Ammo Count: numbers keep changing as player fires ----- Retainer Box?
**Combo Counter: **Also keeps changing as player attacks ----- Retainer Box?
Weapon Image and Name: Will change every time player changes weapon ----- Retainer Box?
Attack Buttons and Progress Bars (Health) ----- Retainer Box?

If anyone could help explain this, I would really appreciate it.
Many Thanks