What is the difference between armor and rating?

The description of every armor in the game says something like “97 Armor (Rating 54)”. The first part is clear, when I equip this armor, my (confusingly named) Armor Rating increases by that much. But what exactly does Rating mean?

Solution 1:

Rating is the quality of the item (explained here under item rating)

Equippable items have a rating indicating their general level of quality. This number gives you a rough idea of how powerful the viewed item is compared to other items you’re wearing. To raise the rating, look for certain modification items that can increase these ratings, and attach them to your equipment.