First, however, I would like to know why you attached a p and an m to the intervallatic reference.
A suspension is beter viewed as a melodic device.
Trying playing a 1 - 4 - 3 melody and put 2 beats on the 3rd...
...Its got a pretty nice sound to it right?
I'm pretty sure you don't hear alot of 1 - 2 - 3 runs on the guitar since the phyiscal lay out of the instrument isn't accomidating to that type of fingering (at least to my level of knowlegde, I don't have time to study everything!) However a 1 9 (being the octave of 2) 3 - that has a nice jazzy, or even "dave matthews" type of sound to it (it depends on how you voice the chord.)
The reason that you have sus2 and sus4 chords is because you can't have a scale without having the chords (it would be like having letters without words.)
A good, or common use of sus chords is to add and then resolve tension - a common arrangment maybe Esus4 to Em. You typically don't end on a sus chord, but who knows - it could sound cool, and its been done before.
You could elaborate: | Em | C | Dsus4 D | Em
You can also use the sus4 or sus2 sound to simply alter the sound of the chord.
The most practical use of these chords is when playing scales and chords at the same time (theres a name for this technique in The Blues and I can't remember what its called, but its refered to as a style.) Also this type of playing is found common place in classical, and metal (the use of pedal points in metal, to where classical music pre dated this terminology.)
Now your asking how they fit in harmonically - I will TRY to explain this...
When you look at a scale (say for instance, a box pattern) take the G Major scale in 2nd position (also sometimes called first position for those who are retarded.) It would actually be a 1st postion F# Locrian Scale (but this is a different theory.)
Back to the subject:
When you look at this scale you should be able to imagine the chord shape in the scale (for example the barr shape.) Because the intervals define the scale, they also must define any chords from which the scale is built. It all kind of fits together like a puzzle.
A G Major scale, is going to build a major chord with a harmonic structure in conformity with ionian intervallatic structure. A G Sus4 derived from this scale must also conform to the G major (ionian) structure if we are to retain definition of G Major.
Essentially its rearranging your fingering so that this new chord structure maybe fretted, in harmony with G major.
Now, If you were to change your perspective, and say that you are now playing a 1st position F# Locrain scale (the same scale as 2nd Position Ionian, but with a different bass note as the root, and different compositional rules applied to the melody,) Then the same physical interface (the same box pattern) applies to the fingering, but the intervallactic relationship which we are using to define the sus chord changes:
1) When we were calling it G Major the interval relationship was a Major 1st, defined by a Major 3rd, and perfect 4th.
2) The interval relationship contained in a Locrain Scale, used to define its corresponding sus chord is: Minor 1st, defined by a Minor 3rd, and a perfect 4th
3) They are the same intervals really - a 1st 3rd and 4th, but have a voicing applied to them depending on the whole step half step, or tone semi tone relation
4) A suspended (sus) must fit or conform to the scale. So an F# Locrain Sus4 run would consist of a Minor 1st (defined by the major 1st of the Key, which is G (count the interval relation between G and F in order and you will see that its a minor 7th) and also defined by the minor 3rd interval relation between the 1st and 3rd (you can't have a minor interval relative to the key without the interval itself having a minor third (like wise with a major relationship))) and a perfect 4th (if you look at the box pattern in that position you will see that it is infact a perfect 4th or minor 2nd (cause these are the intervals that are found in locrain.)
5) Read 4 over and over again until its clear - that is the primary concept
6) Hope this helps...
