Unlike pressure pipe, which is rated by the pressure it can take, such as PN10 and PN16, or for imperial sizes Class C, Class E etc; drainage pipe is classified by it's crush resistance, more correctly called it's "Nominal Stiffness" of "SN". This is because drainage pipes are not subject to internal pressure but can be subjected to external pressures from backfilling when installed underground. The backfilling around the pipe can have the effect of squashing the pipe to an oval shape. This can impact the pipe system in two ways, firstly by weakening the pipe (a circle being the strongest shape you can get) and secondly by leading to leaks around joints and fittings. Piping with a rating of SN4 or SN8 have a strength suitable for burial, and so can be used above or below ground. Installers sometimes prefer to install an SN2 pipe above ground, as it has a thinner wall and so is sometimes cheaper. the thinner wall can however make the exposed pipe more prone to physical damage, which in my opinion, offsets the small additional costs of the thicker walled SN4.