It is an acid fixer containing, according to the packaging, thiosulphate. I am almost certain it is non-hardening as I think Agfa used, at one time, to sell a seperate hardener.
The box says it is a "normal or rapid" fixer, giving time for both normal & rapid fixing and for both films and papers. For instance fixing paper in rapid mode at 20 degrees C (68 F) requires 30 to 60 seconds. To use as a "normal" fixer the solution is diluted more than when used as a "rapid" one.
The 500ml bottle I have is 15 years old, unopened, kept in the dark at average temperatures. Wonder if it is still useable or should I chuck it away?