when do you call a 11 chord a sus4 chord?

[Ron Stern]

In major keys, a (major triad with) 11 is a dominant chord, containing or implying a minor 7, whereas (major triad with) sus 4 is a tonic (unless context indicates otherwise), so they're two different things (though an 11 is sometimes notated as 7sus4).

What justifies saying that one thing in two different contexts is two different things? A tree in front of a house and a tree in a park are two different things?

[JohnRoss]

It's not complicated, Ron. They're two different things because they are not the same, excuse the flippant tautology. An 11 on a major triad contains or implies a minor 7, it's a dominant chord (notes in square brackets probably not played, but aurally there - C [E] G Bb [D] F). If there is any kind of 7 involved with a sus 4, it's going to be a major 7 (C [E] G [B] [D] F, unless the minor 7 is specifically stated, in which case it's an 11 pretending to be a 4 just for the heck of it, or because that's what Mark Levine wants to call it). Why is it a major seventh over a sus 4? Because in major keys, you only get a (perfect) 4 plus a major 7 on chord I. IV has a maj7, but the 4 is not perfect but augmented.

(And an augmented 4 lacks that downward pull to the major third that is characteristic of the sus 4 (if anything, an augmented 4 pushes upwards), whatever, it doesn't have the same "hanging" quality that a sus4 does.)

[Helgo]

What justifies saying that one thing in two different contexts is two different things? A tree in front of a house and a tree in a park are two different things?

Ron,

about those trees: whether they are the same thing or not; discuss this question with a philosophy major over a cup of coffee and you're in for some fun

As far as music and music theory is concerned: yes, things are'nt absolute and yes, they are different things in different contexts. You will have to get used to that, i am afraid.
Want an example?

Look at a Cmin7 chord. This is a different beast in the two following contexts a) and b):

a) C min7 F7 Bb maj7

b) D min7b5 G7 C min7


Even if you use the same voicing: not only the theory, but even the sound is different...
It's music, and it's complicated and it's beautiful

Best,
H.