makefile used by the sub-make makefile!

[ali hagigat]

5.7.2 Communicating Variables to a Sub-make
These variables are defined in the sub-make as defaults, but do not
override what is specified in the makefile used by the sub-make makefile
unless you use the
‘-e’ switch (see Section 9.7 [Summary of Options], page 96).
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Would you please clarify this sentence more? It is confusing. Sub-make
means when we have a command, 'make' inside a makefile, isn't it? What
is 'makefile used by the sub-make makefile'?!
Thank you.