Nathan is close -- they actually show the index levels, with certain upgrades requiring a particular level.
For example (from the link he posted):
Advanced Logistics Network
This strategic upgrade opens up the option of anchoring/onlining Jump Bridges at Starbases within the system. Eligible for installation in IHs within systems with a Strategic Index level of 3 or higher.
If you see a system with Strategic 3, it means there may be a Jump Bridge installed. On the other hand, if you see a system with Strategic 2, you know for certain that there are no Jump Bridges in the system (at least, none that are currently onlinable).
Amnyways, the critical question is, how are those levels determined, and that's covered in this relevant bit (again, from Nathan's link):
Strategic Index
The Strategic Index measures the amount of time during which sovereignty has been held over the system by the current sovereignty holder. The Index is measured on the following scale:
- Level 1 = Sovereignty held for 7 days.
- Level 2 = Sovereignty held for 21 days.
- Level 3 = Sovereignty held for 35 days.
- Level 4 = Sovereignty held for 65 days.
- Level 5 = Sovereignty held for 100 days.
Military Index
The Military Index measures the amount of NPCs killed in a system.
Industrial Index
The Industrial Index measures the amount of ore mined within a system.