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Yes, memo field is for tagging your transaction to help you memorize details on what it was made for, etc.
That's when you are sending your transaction from bittrex, poloniex, blocktrades, etc.
You might notice that when you are sending to such exchanges they require you to put some special id there (never your key / secret). It's for the same reason, so they would know what they should do with such incoming transaction (match it to your account / request on that service)