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Ideally a sectional garage door is fitted to the inside face of a garage opening, but many people don't have an inside return to fit to, or maybe none at all and very deep reveals too, such as on a twin garage opening with a larger central brick pier.
Many brands have multiple standard sizes to choose from for the fastest delivery and best prices in the UK. When installing behind your garage the ordering sizes should ideally match the aperture dimensions, but rarely is this the case.
So, for example a sectional door ordering size of 2286mm x 2126mm is a standard size for many brands (sometimes expressed as the imperial size 7670). If your garage opening was 2300mm x 2150mm then this door would still be fine but would be showing the fixing sub frame partially all round. This is when you need the frame to either match the door colour ro be to a colour of your choice, so be aware of this.
Alternatively if you are installing inbetween your garage opening then of corse the ordering size PLUS the outer sub frame defines the overall width and height required and your garage opening need to be slightly larger to accomodate this with some tolerance. If the opening is quite a bit larger then your packing out and additonal support might be more along with cladding to the sides and head. This is up to you to decide whether you think this is OK or not but possible none the less.