Octa or 500 modules - what's the difference ?

Octa modules and 500 modules have the same dimensions, only the connector is different.
500 modules use 15-pin connectors. Octa modules have 18-pin connectors.

The first 15 pins of the 500 or Octa connector are exactly the same, and are used in the same way as the original 500 series rack.  Difference is in pin 16 to 18. On the Octa connector these are used for +/- 24 Volt power rails, allowing Octa modules to run on +/- 24V
Octa slots and modules are fully compatible with standard 500 series compliant modules.
You can use 500 modules or Octa modules in any Octa slot.
Octa modules can only be used in Octa slots.

Interphase Audio modules are available in Octa and 500 series format.

What is the advantage of using Octa modules? 

Octa modules run on higher voltage than standard 500 modules (+/-24V instead of +/-16V). 
Because of this, Octa modules have extra headroom and better signal-to-noise ratio, resulting in much improved sound quality. 
Octa modules outperform all 500-series modules.

What is the advantage of using Octa Rack / Slots? 

Octa Rack is about the best rack you can get for any Octa / 51x / 500 modules.
Any module, be it 500, 51x or Octa will sound better in the Octa Rack. 
Octa Rack has dedicated voltage regulators for each slot, ensuring each module gets the cleanest +/- 24V 
Octa Rack 500 adds dedicated voltage regulators for the +/- 16V rails, ensuring each 500 module gets the cleanest power.
These voltage regulators filter out all the “dirt” on the power lines and guarantee the cleanest power to each module, resulting in much better and more transparant audio quality (they really do).
The result is that any 500, 51x or Octa module will really sound better in the Octa Rack than in any other rack.
(and the Octa Rack does not “hum” or “hiss” or “distort” like so many other 500 racks do)