A kitchen produces heat from the hob, the oven and the dishwasher, so glazing in a kitchen has a different job from glazing in a bedroom. It has to let light in and also cope with a warm, steamy room.
We look at the glass specification alongside the position. The thermal numbers differ by type. The thermal limits differ by unit type, and the quote names the figure that applies to yours. A solar control coating goes further, trimming the heat that arrives on sunny afternoons.
A kitchen with a south-facing slope will warm up fastest. In that case we may suggest a solar control coating, a blind or both. On a north slope, the light is steady and cool, and a plain clear glass is usually enough.
Safety is part of the specification as well. Glass over a work area should hold together if something strikes it, so the build-up of the panes goes in the quote for you to read. Ask about it at the survey if it matters to you, for example if children use the kitchen or a tree overhangs the slope.
Finally, think about cleaning. A roof window that opens fully and rotates lets you wash the outer face from inside, which matters on a slope you cannot reach with a ladder. We point out which models do this when we survey.