Frame and Leather Tiers (Backpacks)
Frame Tiers and Leather Tiers are a construct introduced in the Backpacks mod. These describe the quality of a backpack of any size. Everything that has a Frame Tier or a Leather Tier will state so in their tooltip, using roman numerals as tier quantifiers.
The Leather Tier of a backpack describes its storage capacity, with tier 1 being the smallest and tier 3 being the largest. The usage of the Frame Tier is currently unknown, except that it must be equal in all Backpack Pieces used in the creation of a given backpack.