Planed Square Edge (PSE) Timber, also known as Planed All Round (PAR) Timber.

PSE/PAR timber is board that, As the name suggests, has been planed on all sides to remove that rough outer layer and give a clean finish with no saw marks. The planing process allows the grain of the wood to show through and reduces the chances of splintering. PSE/PAR timber is ideal for jobs where the wood will be visible such as in furniture and shelving. 

Typically PSE/PAR timbers will be listed with a Nominal Size (the size of the starting board) and a Finished Size (the final dimension of the board).