So, If you open a new, blank document and look at the Resource Browser, there will be nothing there. Import a Spotlight Lighting Device, and look in the Resource Browser and no there are a number of items.
The symbol for the body helps keep the document smaller, instead of repeating geometry, you're referencing geometry already in the document. The things which make a Source 4 575w 19° different from a Source 4 750w 36° are the Record Formats which are associated with the Spotlight Lighting Devices.
These are the things that make anything a Spotlight Lighting Device. If you insert an instance of a Spotlight Lighting Device Symbol, select it and go to the OIP>Data Tab you'll see that Vectorworks has assigned a unique number identifying the unit and that the data records are associated with the symbol instance.
But you can't do anything with the record data. In fact, you can see it.
If you right-click on the symbol in the Resource Browser and select 'Edit 3D' you'll see there is associated Light Info data, but mostly blank. Go Back to Edit the 2D component of the symbol...
Here you can see specific information, AND CHANGE that information about that light. So, if you have a symbol for a 575w unit and you're going to change the lamps to 750's, you can modify your symbol, or dupe the symbol and create a symbol specifically for the 750w units.