BCD and Drysuit Inflators are pretty simple valves that most divers can figure out but not all valves are the same or universal which can cause confusion, especially when trying to buy replacement parts.
There are three common types of Quick Disconnect connection; DIN, CEJN and the larger Buddy connector. There are also other QD fittings around that are less common that I'll try to add to the list as I get information about them.
Now they all have different names depending on where you're from and what your LDS calls them, which makes it four times as hard to find the exact one you need. They all have cut-off valves that close and shut off airflow when disconnected so you don't have to worry about loosing gas if you disconnect.
DIN / International / Seatec / Hanson
This is by far the most common QD fitting, it uses a simple Shrader valve in the LPI hose that stops the airflow until the rod is pushed by the male part of the coupling when you connect the hose just like the valves on the tires on your car.
Schrader valves inside the hose come in different sizes so mixing brands on hoses and inflators doesn't guarantee a functioning pair. For example a Scubapro LPI hose will always work with a Scubapro inflator because the Schrader valve has been selected to work with the Scubapro nipple. Trying to connect a Mares LPI hose to a Cressi inflator might not work because there is no 'universal' standard connector between all the brands. Independent hose manufacturers like Miflex have one of the most universal fittings so will work with most fittings no problem.
Below lies the problem as all of these nipples us the same DIN style QD hose but as you can clearly see they are all different lengths and designs because they are designed to work with a certain type of Shrader valve. They all work in similar fashions by pushing the rod on the Schrader valve but some have short barrels with a bridge over the end, others have longer barrel without a bridge and others have an internal rod that open the Shrader valve. Because of this you can't always guarantee a 'standard' hose will work because there is no 'standard' but there are some that are more 'universal' than others.
Now you can swap Schrader valves and nipples pretty easily and find a combination that works but again as you see above; not all of the threads are the same size and some have sealing O-Rings in different places.
CEJN is a Swedish manufacturer that specialize in Quick Connect couplings. The 'CEJN' coupling is easier to connect as you don't have to retract the collar like the DIN fitting and tend to have a larger collar so you can disconnect easily too. These are easier to disconnect too as you don't have to push the hose in and pull the collar back at the same time, that pressure helps you disconnect with a satisfying pop.
Functioning in a similar way to the DIN fitting; a spring loaded valve closes until connected to the inflator, once connected to the nipple air can flow freely through the hose. I used a CEJN hose on my drysuit for a few seasons and never had a problem with it, it was quick and easy to kit up even with thick neoprene or chunky dry gloves and even easier to disconnect as you don't have to push the hose in before you pull the collar back.
Now this it where it get's tricky because some DIN nipples can fit CEJN hoses. So this means that you can sometimes mix and match your DIN and CEJN hoses, this comes in particular benefit with more closed cell dSMBs around that you can inflate with a QD hose that can be inflated with either a DIN or CEJN hose, so you don't have to fit extra hoses just so you can inflate your dSMB.
The 'Buddy' / Auto Air / Air2
This is the largest of the three most common QD fittings in Scuba and it is made to be able to deliver enough gas for an alternate air source. Used on the Scubapro Air II, Atomic SS1, Buddy Auto-Air they are a quick disconnect hose for your BCD but have a high enough airflow to breathe effectively from.
Similar to the CEJN hoses these are a quick push-on and easy disconnect but they're a rather unique fitting that be quite hard to find in remote areas of the world as they're not widely used so replacing them can be tricky and they don't fit many other couplings so it's a pretty specialised connection.
This all came because somebody asked why their Air Horn connected to their BCD hose and worked but they couldn't add air to their BCD... The connection can fit and attach but that doesn't mean it will activate the Shrader valve. As I said earlier you can take out the Shrader valve and swap it with a different one to make it work but you'll need a tool to get it out and fit it.
When I was teaching students I was showing the difference between the hoses and how they don't fit one-another until I connected a CEJN hose to a DIN fitting and it worked, to my surprise, so I started to experiment with compatibility and some nipples can fit different types of hose and function properly.
If you know of any other types of QD fitting that aren't on my list or names of fittings that you call the ones above pop them in comments below and I'll add them to the list for a more comprehensive database of all QD fittings to help others understand the differences and compatibility.
If you're looking for a hose to replace your regulator hose check out my other blog on which size thread you might need.