Whereas CUL (See You Later) implies that a person is planning to speak in the future, CYA is less formal and does not imply that a discussion will be held at a later date. It is lighter and more jovial than the word bye, and thus ideal when conversing casually. Bye may be more definitive, whereas, CYA retains the goodbye light. One of the reasons why people use CYA is to terminate a conversation in a casual manner that does not make it appear serious or committed to continue talking in the near future.
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