Terminology

Some of the terminology used.

Build system boundary

Refers to when a child directory of a module / project is managed by a different build system.

Example:

.
└── outer
    ├── build.leaf
    └── inner
        └── CMakeLists.txt

And with inner subdir-ed from outer

// outer/build.leaf
project('outer')

subdir('inner')

Here outer is managed by leafbuild while inner is managed by cmake. We call outer the outer directory and inner the inner directory across the ("outer", "outer/inner") build system boundary.