[yocto] Where does the bitbake get the variable "TOPDIR"

[Biao]

Greetings, 


I am a newbie tying to understand how bitbake works.
There is one line as BBPATH = "${TOPDIR}" in the mybuild/conf/bblayers.conf, i would like to know where the TOPDIR is defined? 


Thanks,
Biao
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