Where is the System ID list?

Where do I find the system ID list for all the possible system IDs that would be in the drop down menu of MAVLink Inspector → Analyze Tools page? The drop down would usually be placed on the upper right hand corner, left of the pop out button. A specific location in the source code? Thanks in advance!

This is a picture of the Analyze Tools page.

This it the popout button I was referring to.