I took the occasion to check what is the mechanism of overriding.

Below what myself found in Bitbake User Manual, ch. 3.3. Conditional Syntax
(Overrides) "You can use OVERRIDES to ... conditionally append or prepend
the value of a variable."
This is the point where I wonder why overriding is discussed in context of
append and prepend.
Which element of appending and prepending operations has the nature of
overriding other things?

I use Bitbake user manual mentioned chapter as knowledge source to get more about override mechanism
as OVERRIDE variable description in YP Reference Manual refers to that document for more details.