Simpel answer:
- bikes/scooters/mopeds "low-rev" engines (not really true today...)
- trimmers, "very high revs".
E.g.
* JLO 370cc engine operates at 3000rpm = any engine oil will do as "2-stroke oil"
* Stihl/Husquarna etc 37cc trimmer engines operates at 10 000-14 000rpm = only special 2-stoke oil for high rpm engines will lubricate the bearings and especially the piston and barrell. Poor oil-film/-quallity will brake... and when it brakes, a new engine will be needed...
2-stroke oils were once simple... today, its a ... mess.
http://www.shell.co.uk/home/content/gbr/products_services/on_the_road/oils_lubricants/advance/product_range/two_stroke/http://www.shell.co.uk/home/content/gbr/products_services/on_the_road/oils_lubricants/advance/product_range/two_stroke/#subtitle_12 STROKE ANIMATION - click on the 2-stoke tab, it opens in with the 4-stoke animation:
http://www.shell.co.uk/home/content/gbr/products_services/on_the_road/oils_lubricants/advance/getting_more/I am sorry that I simply can not find the data-sheets/folders from Shell any more - really messy homesite!
Anyway - read up very carefully on the quallities of the oils you are considering!
Call the manufaturers - ask for advice and recommendations.
I do recommend that you use the best 2-stroke oil available; costs a bit more, but saves the engine!
On engines labled 1:50 I use 1:40 and on those lables 1:40 I used 1:30-35. On older engines lable 1:20 I use 1:20 since the moderns 2-stroke oils for high-revolution engins are so much better than the oils used when those engines were manufactured; on low-revovution engines I may even use 1:30 instead of 1:20 (like on theJLO 370cc and the like from the 50:ties and 60:ties up to the mid 1980:ties).
Just be carefull.
It is better to use 1% more than recommended - gives a bit of blue-smoke but saves the engine.