The C7sus4(b9), shorter C7sus(b9) or Csus(b9) is a 7sus4 chord with an additional minor 9th. The 5th of the sus(b9) chord is usually omitted on the guitar.
The b9 (chord extension) can not be written as b2 (modification of the basic chord), although they represent the same tone Db. The chord tones for comparison: Csusb9: C(1) F(4) G(5) Bb(7) Db(b9) Csusb2: C(1) Db(susb2) G(5), 3rd replaced by b2.
The sus(b9) chord is an interesting replacement or addition to a dominant 7th chord that resolves from V to I, for examle in a II-V-I progression like Dm7 G7sus(b9) Cmaj7. Other common chord progressions: Gsus(b9) G7(b9) Cmaj7,
Gsus9 Gsus(b9) C, or
Since the 5th is often omitted in this chord, it is often interpreted as slash chord: instead of Csus(b9) you can write Bbm/C, a Bb minor chord over C as bass note.