A barcode is the last seven nucleotides of a hairpin in a lab submitted design that EteRNA uses for identifying your design. For that reason, it is important that your barcode hairpin sequence is "unique". Not just among the lab, but among all the sequences that will be synthesized together. The barcode region is marked with rounded barcodes as in the screenshot below. If you try to submit a sequence with a barcode that exists in other submitted sequences, you will get a warning message and won't be able to submit your sequence.
<img title="Example Barcode" src="http://eterna.cmu.edu/sites/default/files/chat_screens/10_1358979415.png" alt="Barcode" width="609" height="437" />