Orthos |
I typically made it "any hex that has a location in it that gives a resource". So the Sootscale mine (silver), the gold mine, the fangberry patch, the Mud Bowl (cosmetic and alchemical supplies), the hot springs, etc. etc. etc. Those kinds of locations on the map that provide the PCs something they can use to equip, invest, or produce in their kingdom.
As Tonyz said, no reason you can't say "this hex has something that's not on the map because I wanted to put it there, or you came up with an inventive use for something you found that makes it worthwhile to harvest".