Why In CW traffic handling, AR (sent as one run-together character, di-dah-di-dah-dit) means "end of message." It closes out the formal message text itself, after which the operator may still continue the contact to send more messages or exchange comments. The overbar in written form shows the letters are keyed as a single character with no space between them.
Watch out SK is the tempting one, but it means end of contact, sent when the whole QSO is finished, not just one message. BK means break, used to hand the transmission back mid-conversation, and KN invites only a specific named station to transmit.
AR = end of message, SK = end of contact. Message comes before the whole QSO ends, and A comes before S.