Ah, suspensions.
In 'classical' theory, they don't exist. All chords are built on thirds of some type, so without that root-third-fifth structure, it ain't a chord.
That's not to say composers didn't use them - just that they are not recognized as chords; they don't add anything to the harmony.
The typical use of a suspension is to have a chord, like F major, followed by another chord, like C major. Instead of having all the notes move at once, you move them one at a time... F-A-C becomes F-G-C becomes E-G-C.
The chord change is still recognized as F-C in harmonic analysis, but in between there's a 'suspension' - a term from harmony that means there's a note that's too high for the chord, and it's going to resolve down to a chord tone. If it resolves up to a chord tone, it's called a retardation.
When suspensions started to be used a lot in popular music, the term 'sus' started popping up in sheet music, as in 'Csus'. It ALWAYS meant a suspended fourth (C-F-G). Some publishers then started labelling them 'sus4'... which led to the misconception that you can replace the third with any note and you get a suspended chord. I see references to 'sus2' all the time, and sometimes 'sus6'.
There's no need to use those names - Csus or Csus4 is the same notes as Fsus2, so I prefer it - it's clear. Resolution doesn't matter, since harmony doesn't recognize this as a chord change. I wouldn't call it a substitution, because it's the same notes... it's like substituting Am7 (A-C-E-G) for C6 (C-E-G-A). You get the exact same notes, so you might call it a renaming, but not a substitution
