I think it means anything with -1 can appear on a planet without needing any exploration upgrades. 1 means it can show up on the first exploration upgrade, and 2 would need both ranks of the upgrade (if min is set to 2). I suspect most of the bonuses are set to min 1 max 2?

About the level of upgrade : -1 mean that a scout exploring the gravity wheel is enough, 0 mean that colonize a planet is enough, 1 mean that you need "Explore Planet" level 1, and two mean that you need "Explore Planet" level 2

by example, -1/-1 can be something who modify the gravity wheel like "Super Dense Core"... this is directly feel by an scout or any ship entering the gravity wheel...

a 0/0 can only be discover when colonize... a 0/2 can be type 0, type 1 or type 2... what it will be is chosen in a random way... Artifact are always type 2...