Does a backwards K signify solely a called 3rd strike or do all 3 pitches need to be called strikes?

A backwards K specifically signifies a called third strike, meaning the batter did not swing at the pitch that resulted in the third strike. It does not imply that all three pitches were called strikes; only the final pitch that resulted in the out is indicated by the backwards K. The regular K, on the other hand, represents a swinging strikeout.