According to Gulenko 1,8,6,3 are strong functions. 2,7,5,4 are weak functions. Functions that match your vertness are strong. Those that don't are weak.
erm, strong/weak isn't the best terminology.
2 isn't really "weak," it's implicitly aware of a lot of information, but it requires tremendous energy (discipline ?) to use. This is one reason illusionary partners can get tired of seeing each other.
7 thought process is entirely subdued, but it's aware of lots of information on a mostly verbal (syntactic rather than semantic) level, allowing the person to ape its use in certain circumstances.
5 and 4 are obvious.
8 is very strong. 1 thinks up something and transfers it to 8, which then tries to mold the output according to situational factors. It's unvalued because nobody values situational factors that glut up your thought process -- but you still have to be aware of them. Spending time with your dual is rewarding because you don't have to focus on situational crap that interferes with what 1'd rather do.
Also you use this function privately to shield your dual from their mistakes, blah blah blah.
3 is strong but not really. It often manifests itself, but in a puerile or childish format. For example, an IxE that engages in petty territorial games, over the top displays of confidence or crude sexuality . (It doesn't have to be that assholish, I'm illustrating a point). It manifests when 1 has been fully saturated and your fried brain automatically switches to another function.
6 is sort of obvious. 1 is your weltanschauung; Nuff said.