where is document for Rule Placeholder variables like <SOURCE>

Code
1

I see there are rules with placeholder variables likes

set(CMAKE_C_COMPILE_OBJECT "<CMAKE_C_COMPILER> <DEFINES> <INCLUDES> <FLAGS> -o <OBJECT>  -c <SOURCE>")

Though i can find it in source Source/cmRulePlaceholderExpander.cxx,
I can not find out any manual about <DEFINES>, <SOURCE>, and etc on https://cmake.org/cmake/help where I hoped it be.

Any help will be appreciated !

2

AFAIK, these tend to be treated as internal implementation details and aren’t really documented. If anything, they’re actually useful to toolchain file maintainers, but even there, they’re usually “figure it out once and then leave it alone”.

Cc: @brad.king

3

In my team, they just copy from each other, and make some mistakes here and there.
Maybe somebody figure it out once and then left for new job, so sad!