Destination Tag when receiving XRP:
When you withdraw XRP funds from a XRP exchange to a wallet you can fill the tag field with any numeric ID you want to identify yourself as the recipient of the XRP transaction.
Destination Tag when sending XRP:
• When sending funds to an exchange, you are required to fill in a destination tag.
If you do not fill this destination tag, your XRP transaction can be rejected and return to you minus the transaction fees, or your XRP transaction can be stuck in the exchange. In that case you need to contact the exchange.
• Go to the sending screen of your XRP wallet and click on "Advanced mode".
• You copy and paste the tag that the website gave you to identify your transaction.