Fire-rating in sheetrock just tells you how long you have before the sheetrock will crumble
Putting a fine point on it, the UL wall structure ratings are connected to how long a given wall assembly (studs, fasteners, and all the "bits") will delay "burn-through." The concept is that the amount of time represented by the protection value is how long users of a given space have to discover the fire, and have it dealt with and/or escape to safety.
All drywall products contain some amount of entrained water vapor, which will outgas at a given fire temperature, and will, as a result, not burn-through. The sheeting materials (paper or similar products) will prevent the gypsum inner layer from crumbling until such time as the materials are consumed in a conflagration. The paper can be treated with various products, like borates, to limit flame propagation or spread, or charring and smokiness. (There are endless tedious tables for smoke spread and the like based on which product is used.)
Whether or not a given wall will fail during or after a fire, is a separate structural equation, as UL does not rate load-bearing abilities.
These are the endless picayune details those of us in Architecture must needs know as everyday knowledge.
It's part of how I know I can detail and specify a safer room than the average RSC represents for firearm protection (or human protection, come to cases). Rooms are not nearly so easy to shift around than RSC, though. An armoire can be better than a closet, and vice versa.