For low output low rpm alternator is usually mentioned about lower than 100kva alternator with rpm is lower than 1800rpm for 60Hz and 1500rpm for 50Hz. This type of alternator is used for small diesel generator with a very basic diesel engine.
For high output low rpm alternator is usually mentioned about higher than 1000kva alternator with rpm is lower than 900rpm for both 50hz and 60Hz. This type of alternator used with very special engine type with the same rpm and engine fuel is usually DO diesel fuel.