UV (C), (N) and (O)??

[Razor-BladE]

So i'm looking at Hoya UV filters and some of them have either (C), (N) or (O) after 'UV'. What does this mean?

 

[J1234]

Iirc it's to do with the coatings on the filter and where it was made.

 

[Razor-BladE]

Do you know if one is better than the other?
And i heard that i want to be avoiding slim frame filters?

Edit: Found out what (N) and (O) are:
(N) means 'normal' glass, and (O) means optical glass.. so the (O) ones are better. But no idea what the (C) is lol.