EMBED_MANIFEST_AT is a Makefile variable which specifies that, on Windows, this code/module should have a manifest embedded at the specified resource ID.
Tree Makefiles set
EMBED_MANIFEST_AT=N before including
Setting this variable has no effect on non-Windows platforms. By default, DLLs do not get a manifest embedded in them, because they are loaded using the application's activation context, and EXEs do get manifests embedded properly, so DLLs do not need them. The primary use case for this is DLLs that are expected to be loaded by external processes, such as the accessibility modules.