sus & add chords
sus4 (sus), sus2
In a sus chord the 3rd is not played (it is "suspended").
- In a sus4 chord (often just sus) the 3rd is replaced by a 4th
- In a sus2 chord you play a 2nd instead of the 3rd.
Since sus chords have no 3rd, they are neither major nor minor, but form an own category.
7sus4 (7sus), sus9
sus9 is just an abbreviation for 7sus4(9) - a sus4 chord with a 7th and a 9th.
The 9 actually represents the same tone as the 2 in the sus2 chord. The different numbers in the chord symbol don't tell us, where exactly the tone has to be played on the fretboard. They just help to identify the other intervals of the chord. 9 means: "there is also a 7th", 2 just means: "the 3rd is missing".
In an add chord a tone is added to the basic triad. While C9 also contains a 7th - it's an abbreviation for C7(9) - there's no 7th in a Cadd9.
Thus a Cadd9 is a C major triad (C E G ) with a 9th D.
On the fretboard...
On the staves you can see the theoretical form of the chords. In a Csus4 chord e.g. the E will be replaced by an F. However, practically that F can be played anywhere on the guitars fingerboard. You might play it an octave higher and there can also be more than one F in your chord shape (you have the same freedom for all the other tones in any chord).
Root on A-string: example chord shapes in C
Root on E-string: example chord shapes in A
Example chord progresions for sus and add
Here are five progressions in different keys. First you can always see a version with major, minor (also some m7 and maj7) chords. The variations (button) show the current progression with the use of sus and add chords.
Arguable (?) chord symbols
The intervals of the basic chord are named with numbers up to 7 (e.g. 1 m3 5 7 for a m7 chord or 1 3 5 6 for a sixth chord). For additional tones an octave is added to the interval (only in the chord symbol!), which results in a 9 instead of 2, an 11 instead of a 4, a 13 instead of a 6.
So if you stumble upon an add2 chord, it should probably rather be called add9. Add4 should better be called add11. It's different in a sus2 or sus4 chord because the 2 or 4 replaces the 3rd and thus becomes part of the basic chord. If the composer wants you to play the 9th exactly a 2nd above the root, he should better write out the exact chord note by note on staves or tabs, because chord symbols don't tell you the voicing and consequently don't tell in which octave the 2 or 9 (respectively 4 or 11) has to be played.
The intervals of the basic chord are named with numbers up to 7. Additional tones are named with 9 (b9, #9), 11 (#11) and 13 (b13), unless they replace an interval of the basic chord (2 or 4 replace the 3rd in a sus2 respectively sus4 chord, 6 replaces 7).
Chord symbols don't tell you, in which order/octave the intervals are practically played.
Technically speaking both extensions are correct. Practically one should consider that in contrast to major chords you can add a 7th to minor chords in nearly every musical situation or style without hesitating (e.g. Dm7 instead of Dm). If you play a certain voicing of a minor chord with a 9 on the guitar, it may happen that there is no more string respectively finger for the 7th. Consequently, it will often be appropriate to write a m7(9) [short: m9] or a m7(11) [short: m11] and to just omit the 7th on the fretboard.
Similar situation with sus2 chords: instead of a basic major chord you can play an add9 chord in nearly every musical situation (if you like the sound). If you don't play the 3rd of the add9 chord, you are actually playing a sus2 chord. Since a sus2 chord is just an add9 without the 3rd, you may consider writing add9 or even a basic major chord on your music sheet and leave the player free to play major, add9 or sus2.
The sus2 chord can be seen as an add9 chord without the 3rd and can be played instead of a basic major chord. Sus2 chords are rarely written by the composer. Exception: line clichés* like sus2 → major → sus4 → major (e.g.. "Sommer Of 69" by Brian Adams).
*) A melody is created by changing the chord type (but keeping the root).