Issue Description
Understanding the behavior of Roblox replication – whether objects are ready by the time a script in a certain place has been executed, be sure when to use :WaitForChild(), the guarantees you might have about some stuff loading such as the player character, wether it is guaranteed instances inside of the same container (e.g. PlayerGui) are loaded by the time a script inside that same container has executed… – is a must.
Yet, this collection of information is absent in the community, as I see oversights, mistakes, and uncertainty, all the time.
All of the above assertions should be tested, verified if not already well-established, and documented, as these guarantees really remove some weight off of the developer’s shoulders 
We need an article that encompasses everything you should know about replication, things that this picture sums up very briefly
and also covers some obscure behavior that you only find mentioned in some announcement posts such as: Changes to Non-ReplicatedFirst Loading Order
Issue Area: Documentation Content
Page URL: DevHub | Roblox