This is entirely dependant on which product you’re planning to install. Eg. with upstand or for builders curb.
For builders curb, you work to the external upstand dimension. To suit existing upstand the rooflight sizes are external kerb sizes and must be fitted onto a 75mm wide kerb. For example, the 1200 x 1200mm unit has an external kerb size of 1200 x 1200mm and and an internal size of 1050 x 1050mm.
For rooflights with an upstand, the hole you cut in the roof is the size you order. For example, a 600 x 600mm rooflight will require a 600 x 600mm hole.