Sorry if this has been done already but I'm on my iPhone at work and for some **** reason search won't work!
Anyway thought this may be of some use to people when selecting what gear to wear:
The 2 catogories on a peice of gear (optimal rank and favoured class) work independently of each other. What I've noticed is optimal rank effects the base line stats of the peice like def and Eva.
Whereas the favours part only effect the bonusses received from wearing I.e. +15mp or str +5.
Currently I'm at ARC18 and equipping the trackers body piece (optimal rank 29) drops my def by 3 points compared to what I was wearing. But as peices favoured class is ARC I still receive the + bonus to hp str and dex.
At that point I thought I'll trade 4def for those stat boosts thank you muchly.
Not sure yet as to what the durability/wear situation will be yet. Though I carry all my repair gear with me anyway so no problem.
Hope this helps and hope I ain't double posting!