Different for different countries...... so check your manual
In the USA octane is measured differently. USA 87 is about the same as 91 elsewhere.
In Thailand 91 is benzine / gasoline, and it is mainly used in motorcycles.
The 91 and 95 gasohol (blended with ethanol) that they also sell in Thailand is ok for all motorcycles made in the past 5 or 6 years, but not for many of the older ones. Most cars made since the mid '90s are gasohol compatible.
Ethanol has a lower calorific value than gasoline, so you will go further on a tank of pure gasoline than you will on a blend.
Ethanol is quite corrosive..... and the greenness / ethics of biofuels is questionable. I am not convinced. If you wanna go green, ride a bicycle. I do, a lot.