GNU Makefile ule

Hi Help desk,

I have a xxx.mk file. To this effect there are a couple lines I can't figure out how make command works:

modules := abc
abc_type := static_library
abc_target := libabc.a

I asked the same question to gcc-help and was given some advices to check your manual; especially chapter 6 and 10. At the same time I was asked to clarify further to the right team.  I checked the PDF file and did not find any description like {module}_type/{module}_target rule. My question is whether there is an implicit rule that make cmd will try to find {module}_type/{module}_target variables according to defined modules?

Thanks
Ming