What are Clarks 2P and 4SP cable housings? Lots of websites stock Clarks cable-outers. Lots say which type -- but I can't find what that type means. Can you help?

I suspect 2P is a fairly standard brake cable outer (5mm, spiral-wound, nylon-lined) and 4SP is a fairly standard gear cable outer (4mm, straight-wired, nylon-lined). Is that correct? Are there also other types?
I recognise those numbers so yes that’s right, but don’t ask me to explain what they mean! Buying housings nowadays doesn’t require overthinking, just buy from someplace you trust like anything else. 

You can get compressionless brake housing, used for example for longer cable runs where you might otherwise get a spongy brake feel. It’s similar to gear housing, which must be compressionless, but obviously larger diameter and is a more expensive upgrade option so will get filtered out if you’re searching by price. 

Shimano make a coiled gear housing called OT-RS900 a length of which comes with new R8000 rear derailleurs and probably some other models. Very slick, cuts like butter, but also a bit of a luxury item which you wouldn’t often use.