You can check if a definition is server-side or client-side by grouping by Source from either the Graph module or the Analysis Preview section of the definition details page.
If the definition is client-side, the result will be web.
If the definition is server-side, the result will be server.
if you have two separate events that represent the same interaction, ex. a client-side purchase event and a server-side one, you set up a combo event to view these as one event.