Function of hypo used in fixer is:
An aqueous solution of ammonium thiosulfate (“hypo”) dissolves the unexposed silver halide grains. It forms stable, water-soluble complexes with silver ions, which diffuse from the emulsion. Excessive fixation (hours) results in a gradual loss of film density because the grains of silver slowly dissolve in the acetic acid of the fixing solution.
White and Pharoah's Oral  Radiology Principles and Interpretation 8th edition
Inactivates the carryover developing agent
Dissolves and removes the unexposed silver halide
Prevents damage to gelatin
All of the above