Is there a convention or rule used to name AAA interface in 3GPP?

Question

Appologize if this has been answered.

I'm looking for what Gx, Gy, Cx interface stands for or What does single character mean.

Is there a convention or rule used to name AAA interface in 3GPP?

It looks like IANA is assigning Application IDs. And 3GPP is suggesting it's name. ( listed in https://www.iana.org/assignments/aaa-parameters/aaa-parameters.xhtml#aaa-parameters-46 )

I Searched whole 3GPP web pages and googled 3 days, but I could'nt find anything about naming rule.

So, It seems to be assigned in alphabetical order. but the first letter is likely to symbolize something.

Please understand if the English expression is weird because I am not a native speaker.

Thanks in advance.