What is the difference between fittings?

[CepasCepas]

Hello. Anyone can explain me what is the difference between A, G, S, U, D systems fittings on kegs?
I'm planning to make sparkling apple cider in KEG for first time. Any suggestions what system should I use? Or what is the best? Or there is no differences?
Thanks

 

[CepasCepas]

Can I use used KEG for my cider? Probably it was beer inside in the past. Can yeast from beer it change my cider taste?

 

[Leadpencil]

1.The differences are the breweries. Most U.S. breweries use a D style coupler.
2. As far as which one is best, that depends on availability for you to get.
3. You absolutely can. Just clean and sanitize the keg first.
Here is a list of keg couplings and who uses them.

https://www.kegworks.com/blog/keg-coupler-list/