Orchestra

Well...

the lowest note (in blue) that it says all horns can play is... wow thats low..

(counts ledger lines) ...

G (6 ledger lines down)

and the lowest POSSIBLE note is an E .. 10 ledger lines down.

I've yet to hear a horn player get down to that - even my tuba would struggle.

And that's treble clef, transposed to F, right?  If so, then I can hit all those notes.  If not, then there's never been a horn player alive who could do that.  The horn has a theoretical range nearly an octave below that (F#, I think, is the lowest note), but I can't play there (yet).  My lowest note is a C two octaves below middle C (transposed to F, of course), and my lowest solid note is the D right above that.  On a good day, of course.  I probably shouldn't have told you that.  We aren't supposed to tell composers how low the horn can go, for fear that they might write something using those notes.

yeah it did say Horn in F.. but i didnt understand what that meant.

like for trumpets / clarinets i could change their key signature to match the rest of the orchestra but i couldnt change the Horn, so i didnt know if that was a different situation.

Maybe it let you change the trumpet and clarinet because they're more readily available in other keys (I think).  Horns, however, are almost always in F, often with a key to switch to a B flat horn.

My lowest note is a C two octaves below middle C (transposed to F, of course)

My oldest son plays French horn, and his lowest note is an octave below that (the lowest C on a piano).  He tells me his teacher can play "a little lower than that", but then his teacher performs with the CSO here.

That's very impressive.