There are many different types of plasterboard available for different applications. Stopping finishes the product ready for painting. There are different levels of finishes for different requirements.
Level 5 is a complete covering of all surfaces so a discrepancy between the plastered surface and the non-plastered surface cannot be distinguished, especially in high artificial light, e.g. wall lighting and feature walls and where strong sunlight shines along a wall on a steep angle.
Different gloss levels also affect the level of finish required. Bathroom ceilings often require higher gloss, so a better level of finish is needed. Ceilings often require more attention when high windows cast light of a ceiling to reflect light to the eye. Sloping ceilings are particularly susceptible to this reflection.
Level 4 is used in normal situations where sidelight and critical light are not present. The use of matt and low sheen paints also are factors in choosing in level 4 finish. Some thin high quality wallpapers require level 4 finish to be used. Other lesser levels for textured wallpaper or backing paper can be employed.