All extension chords can be either major, minor, or dominant. If you know your theory: sevenths, ninths, elevenths, and thirteenths are all build on triads. So depending on the triad, you get the names either major ninth, or minor ninth. "b9" or "#11" means you alter that note in the harmony. Also, note that as you go up, all the previous extensions must be in the chord as well. So a major eleveenth must have each the 7th, 9th, and 11th also. Otherwise it is noted as a "add13" or "add6". 11th is written as "sus4" if the 7th and/or 9th is not apparant. An ninth chord without a 7th is reffered to as either "add9", "add2", or "sus2".