How does unity file / adaptive build works when building?

I’m stuck on this issue where one .cpp is not being included in the built, generating an error as a consequence.

0>[Adaptive Build] Excluded from ProjectName unity file: FileName.cpp

I’m not getting any tip or information as to why this file is being excluded and what should I do.

Can anyone point me where can I go to research more about Adaptive Build and the unity file technology? I can then start finding out what is going wrong here.