Robert's right.

The "Optimal rank" is the first job level at which you can equip something without the stat benefits being decreased. "Optimal rank" is determined by your job rank, instead of your physical level.

It's very rare that using a tool or weapon that's got an optimal rank above you will give you better stats than using one with an optimal rank below you. You almost never want to use a tool or weapon that's above your rank. However, you can sometimes get away with using armor that's slightly above your rank.