Getting data from the servers - clarify or further explain what kind of knowledge needed

**Your Godot version:**
3.5, "latest"

**Issue description:**
In "Optimization using Servers:
>Try to never request any information from RenderingServer, PhysicsServer or Physics2DServer by calling functions unless you know what you are doing.

What kind of knowledge is needed? Is it possible to make instructions containing said knowledge? If not, where one can educate himself? What system or peace of code should one explore to gain that knowledge?
>This will severely decrease performance if you call them every frame (and it won't be obvious why).

Built - in functions often get data from servers each frame and multiple times a frame. What's the difference between build in function's calls to servers and user code calls? If there is some sort of system for proper arrangement of this calls, can it be accessed? If not, why? If there is no such system, can preferred call patterns be found in code, by exploring built in functions? Is it good idea to use their call patterns as reference?

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/latest/tutorials/performance/using_servers.html
https://docs.godotengine.org/en/3.5/tutorials/performance/using_servers.html