I'm not sure at 100% but should work like this:
rare astral gear has a 3rd stat in them that can be changed.
For example a wand can have:
2 main stats (vigor + stamina)
2 secondary stats (proficiency + vitality for example)
1 additional stat with a low value (like anger)
This last one can be changed using an Insignia.
You can use Rare Insignia on Rare items, Epic Insignia on Epic...and so on. Obviously you can't use Rare on Epic.
You can use them only on astral gear and not on lower level gear like crafted or reputation.
I'm not sure if you can change an offensive stat like Anger in a defensive like Willpower.