I don't have a formal theory background, but I would say sus2 is a triad and add9 is an extension which comes after you already added the 7th? Sus2 is also a suspension, which means it resolves to a major triad. Am I right?

That is why I was wondering! Because a 9 means that 7 is included just like 11 means 7 and 9 are included.

But some chord sheets that I have have a add9 seeming to mean the same thing as a 2 chord. I thought this was a misleading way to notate it and it turns out I was right. (at least according to this video)

@Mark Tomato Alley
you are confusing a 9 chord with an add9 chord. For example:
C9 would be C E G Bb D.
C Maj 9 would be C E G B D.
And C add9 chord would be C E G D.

i'm not really sure what the logic is but thats just how it is written. to clarify: add9 means you add a 9th on top of a triad (4 notes). 9 means you build it on top of a 7th chord (5 notes) (dominant by default)

A sus chord means you are omitting the third for a perfect second or a perfect fourth. you can still have extensions on top of a sus chord. for example: a 7sus4b9 (a chord with root, perfect 4th, perfect 5th, dominant 7th, and a flat 9th)

also a sus chord can resolve to a major OR minor chord (or neither). i guess a sus2 resolves a bit more cleanly to a minor chord where a sus4 wants to resolve to the major (2 is a semitone away from minor 3 and 4 is a semitone away from major 3). but you can resolve to either. or neither. while thats how they are commonly used, you don't *have* to resolve a sus chord

.@Andrew Shewaga. Thank you

@Andrew Shewaga Nice clarifications, yes!

@Benjamin Jack Yes, it could be that the person who wrote the score didn't exactly know the difference. As @Andrew Shewaga was pointing out, I wasn't clear in my definition: the 9th is a chord extension (the second round of available notes after the first octave has been used), while a 2nd is not.