This is determined by either the synthesis rank or the optimal rank of the item. If both are present, the lesser of these two values is used. If your crafting rank is equal to or higher than the repair rank, then success is virtually guaranteed. If it is less, however, you may attempt to repair the item, but the chance of success decreases for each rank disparity. It is not possible to attempt repairs when this disparity is greater than ten.

That is precisely what this means.

The rank required to repair differing quality versions of the same item does not change. The odds of success, however, are calculated differently. For example, a rank 20, +2 high-quality item is considered as a rank 22 item for the sake of repairs. Attempting to repair such an item at rank 20 will result in a somewhat diminished chance of success. That being said, this "assumed" rank is not the item's true rank, and repairs may therefore be attempted from as early as rank 10 rather than rank 12.

We are currently looking into extending repair NPCs' expertise to the realm of accessories.