Unreal Plugin Language (UPL): What do these UPL tags for GameActivity.java do exactly?

UPL doc : UPL Documentation. Scroll down to line no. 378 - 382.

What does the following tag do exactly?

<gameActivityPostImportAdditions>

What is the difference between

<gameActivityImplementsAdditions>

and

<gameActivityClassAdditions>

tags?

For the last two, aren't they both putting codes in the class?