I assume that is a very novice question but until now, I have been able to do stuff without knowing what these are, mostly because I have tweaked other people's databases without having to interfere with them or did not use them since I don't know what they are or what they do.
In FM Starting Point, they use constant_id. The creator of the db I have worked on at work used a field called commonOne that is in several tables. It is often linking two tables but sometimes exists in a table without being used as matching field of any relationship.
Thank you for enlightening me.