For DDBJ nucleotide sequence submission system (NSSS), you must input nucleotide sequence(s) in FASTA format (for 1 sequence only) or in multi-FASTA format (for 2 or more sequences).
Related page: Format of the nucleotide sequences that you can paste or upload
You must insert the end flag (//) at the end of each sequence when you use MSS for the submission. Please see the page, "How to Make Sequence File".
See also Wikipedia, FASTA format