buildlink3.mk vs builtin.mk

Some of my package's dependencies have a builtin.mk as well as a buildlink3.mk.  Should I include the former instead of the latter in my package's Makefile? Other packages' Makefiles have CHECK_BUILTIN.pkgname set around the include, but I can't find documentation describing this. I could try blindly following the examples, but I'd prefer to understand. Reading the Pkgsrc Guide is not very illuminating on this point.


Thanks,
Tim

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