According to this article, accession codes should consist from a combination of uppercase letters following a combination of digits. If this is a RefSeq, it can have a prefix as a combination of uppercase letters with underscore.
But this accession code 6MWN_B does not consist with all aforementioned. What is it? I could not find an answer via Google.
In general, an accession is a value that points to (provides access to) a single database entry. Different databases (GenBank, RefSeq, Ensembl, SwissProt, and so on) have different conventions for accessions.
The accession in question points to an RNA sequence associated with a Protein Data Bank (PDB) record: "Crystal structure of hepatitis A virus IRES domain V in complex with Fab HAVx". In the PDB record, you can click the "sequence" tab and select chain "B" from the drop-down, and you will see an RNA sequence (ACGU) matching the DNA sequence (ACGT) in the NCBI record to which you linked.
I'm not sure what the rules or conventions for PDB accessions are, but they are clearly different from RefSeq accessions.
