Rough Sawn Timber or PSE or PAR
First the the jargon.
Rough sawn or just sawn is the cheapest and easiest to understand, its roughly sawn to the size stated. It's cheaper because the Timber Merchant hasn't had to do any expensive machining to it.
PSE, Planed Square Edge or PAR Planed All Round, as far as I can tell PSE and PAR are interchangeable. Suppliers use both to described timber that has been planed on all four sides.
What to choose Sawn or PSE/PAR ?
I’ve got a planner thicknesser so I'd have rough sawn timber in most circumstances. This allows me to machine the timber to the exact dimensions I need after its acclimatised, (dried out to the right level). (Another page on this). It’s also cheaper!
PSE/PAR will still need some finishing to get a smooth surface. More importantly when its dried out a bit more the dimensions will have changed and it often distorts requiring planing to get it back to straight and square.
With PSE /PAR the dimensions are given as the nominal sawn size before it’s been planned.
SO BE AWARE You might think you are buying 50 x 100mm but the actual size could be as small as 45 x 95mm.
Having said that, PSE comes in handy if it is the right size off the shelf.
This brings us nicely to timber sizes.