One is the Deep Specialty, which qualifies you to go to 40 metres. (The depth it qualifies you to go to is actually less important than the skills you will learn on the course.)
The other is the Enriched Air/Nitrox Specialty. Enriched air is ordinary air that has been enriched with extra oxygen. This reduces the nitrogen concentration, which is a good thing for two reasons.
It’s not as simple as just putting more oxygen in your cylinder and jumping into the water, however. Oxygen is toxic when breathed under pressure (you just can’t win!) and can cause convulsions. At the bottom of the ocean, a convulsion is bad news. So while you are free to add oxygen to your breathing mix, your maximum depth is restricted by the richness of the mix you choose. Nitrox mixes are referred to according to the percentage of oxygen in the mix. Normal air has 21% oxygen: Nitrox 32 means that the cylinder has 32% oxygen in it.
The Nitrox Specialty is mainly theory – there are some formulas that you need to get to grips with, and you need to understand the two-edged sword that is enriched air. Once you’ve mastered the theory, you’ll learn how to use a Nitrox analyser, and probably do two dives on Nitrox.
As you dive more and more, your air consumption gets better and better. When you get to the point where your dives are limited by the no-decompression limits of air rather than the amount of air in your cylinder, the Nitrox specialty becomes extremely attractive. If you’re in the group of divers (such as older, or overweight) who are most at risk of decompression sickness, diving on Nitrox is a huge investment in your own safety. And finally, if you do repetitive dives (several dives in a day), diving on Nitrox will extend your total bottom time tremendously.