I started working in depth with our synonym (meta data model) editor only recently; there are a lot of neat things you can do with it including adding your own calculated fields, applying friendly names, and casting data types.
One common thing you can do is add a counter DEFINE field to the synonym. It’s important when creating this field to ensure that the “WITH” attribute is set to the segment.
Otherwise, when you add this Define Field in a report on its own, it will give you this error:
(FOC493) NO ACCESSIBLE FIELDS ARE MENTIONED IN THE REQUEST
Here’s a screenshot of how to do it through the Synonym Editor: