All potential bonuses

I think an easier way to understand it is as follows:

The first (primary) bonus of an item always corresponds to the rank.

Unique bonuses: Always appear on the first line of Unique items
Epic bonuses: Always appear on the first line of Epic items
Rare bonuses: Always appear on the first line of Rare items
Junk bonuses: Appear on Rare items on lines 2/3

The second and third line of items are usually one rank below the primary bonus. So for example, on an 3 line Epic item, 90%+ of the time, you will receive these bonuses:
-Epic
-Rare
-Rare

Occasionally, the second and third line bonuses can be better than normal. There is no limit to what rank the second and third bonus can be. So for example, it's possible to obtain on a rare item:
-Rare
-Junk
-Unique (very very rarely)

The only limitation to this is that the Unique bonus must be a better version of a lower level bonus. So, you can't obtain Decent Hyper Body on Rare pants, no matter what, but you can obtain 9% dex.