What does the K mean in airport codes?

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Tags

1. #states
2. #where
3. #airports
4. #prefix
5. #location
6. #reserved
7. #codes
8. #these
9. #united
10. #airport
11. #applicable
12. #contiguous
13. #generally
14. #usually
15. #listed
16. #prefixed
17. #identifier
18. #state