In order to understand what these suggestion modes do, a basic understanding of how Aspell works is required. For that, see Aspell Suggestion Strategy.
The suggestion modes are as follows.
This method will use the fastest method available to come up with decent suggestions. This currently means that it will look for soundslikes within one edit distance. This method will also use the replacement table if one is available. In this mode Aspell gets about 87% of the words from my small test kernel of misspelled words. (Go to http://aspell.net/test for more info on the test kernel as well as comparisons of this version of Aspell with previous versions and other spell checkers.)
This method is currently identical to ultra.
This mode will use what ever method is necessary to return good suggestions in most cases in a reasonable amount of time. This currently means it will looks for soundslikes within two edit distance apart. This mode gets 93% of the words.
Like normal except that “reasonable amount of time” is not a consideration. In most cases it will return the same results as normal. The biggest difference is that it will try an ngram scan if the normal methods of finding a suggestion fail.
This method is like slow but is tailored more for the bad speller, where as the other modes are tailored more to strike a good balance between typos and true misspellings. This mode never performs typo-analysis and returns a huge number of words for the really bad spellers who can’t seem to get the spelling anything close to what it should be. If the misspelled word looks anything like the correct spelling it is bound to be found somewhere on the list of 100 or more suggestions. This mode gets 98% of the words.
If jump tables were not used then the normal option is identical to fast and the slow option is identical to the normal if jump tables were used.