Like many of my solo pieces, I like to use call-in-response as a way creating a sense of dialogue, and what some might call self-interplay. This is very common amongst chordal instruments, but less common amongst wind instruments due to the fact that they're monophonic by nature.
As a matter of fact, this is one of the reasons I began my several-year study and practice of multi-phonics. Mastering them allowed me to expand my sound palette, enabling me to perform similar musical tasks as piano and guitar players. Many of the multi-phonics used on this piece function as upper extensions of the dominant 7th chords.
For example in the first two bars, I establish the harmony by playing a riff that outlines the G chord. Consequently, when I sound the multi-phonic with the Bb, G, and Eb, it's naturally heard as the #9, 8, and b13 of the chord. The rest of the piece follows a similar logic.
Just a quick note about multi-phonics. In the beginning, if they're difficult to play in the context of the piece, I suggest practicing them in isolation. And as a general rule of thumb, the slower the airflow and the more relaxed the throat, the more one can control the multi-phonic. For further explanation about controlling the speed of the airflow, please see my earlier post "Oral Cavity Manipulation."
In understanding the saxophone tablature, the 8va means that the octave is pressed. Everything else, I believe is self-explanatory.
Lastly, as a musical reference, I've included a recorded example which demonstrates how I interpreted the piece.