Below is the current list of measurements that are being tracked for automatic medal assignment. A medal condition includes both the measurement and the amount (which could be 1), so you can create different medals for different amounts of the same measurement.
Measurements are best suited to tracking instant events, such as destroying a ship or completing a survey. More extensive events, such as successfully defending a planet over potentially months of game time or taking part in a major fleet engagement over several days, are not straightforward to track with measurements. These type of major events, covering large areas of space or lengths of time, should be covered by the mass award of campaign medals.
Hostile Ships Destroyed
Military Shipping Destroyed in Tons
Commercial Shipping Destroyed in Tons
Successful Boarding Combat
Hostile Missiles Destroyed
Armour Damage Taken
Internal Damage Taken
Ground Forces Destroyed in Tons
Combat Drop - Transport
Combat Drop - Formation
Ship Destruction Survived
Star Systems Explored
Habitable Worlds Discovered
Ruins Discovered
Installations Recovered
Bodies With Minerals Discovered
Jump Points Discovered
Research Points Generated
Research Projects Completed
Length Of Service in Years
I'll continue to add to the list over time.