SIMPLE ANSWER? When the global field is the parent in a relationship.
In any relationship, the record on the parent side is a single value that is the same in a matching field in one or more child records. Think of an invoice and line items on that invoice. The invoice number, say 1003, would match the contents in a field in the line items table (the foreign key). Each foreign key in the related child records would have the value of 1003. Now, you do not want to use a global field as a unique ID in the parent table. That would result in all parent records having the same set of related line items. Not a good thing.
But it’s great to use a global field as the primary key when you want to display different sets of related data on the fly. Change the contents of the global field, and a portal of related data shows up right away.
Find out more about using a global field as a key field by reading Dwayne Wright’s blog post on this subject. If you are learning FileMaker, bookmark Dwayne’s blog, FileMaker Thoughts, and visit it often.