An accession number is the unique id EDGAR assigns to every filing when it is submitted. Listed shows it in the note beside every figure, so you can open the exact filing the figure came from.
Format
Eighteen digits in three groups:
NNNNNNNNNN-YY-SSSSSS
- NNNNNNNNNN: the CIK of whoever submitted the filing. That is often the company itself, but it can be a filing agent working for it, or an insider filing their own Form 4.
- YY: the year of submission.
- SSSSSS: a sequence number for that submitter in that year.
So the first part of an accession number does not reliably tell you which company a filing is about. The filing's index page does.
Finding the filing
On sec.gov, a filing's documents sit in a folder named after the accession number with the dashes removed, under the company's CIK. Listed's filing pages live at /filings/<accession> and link to that folder.
Why it matters
Companies amend and restate. The same period can have several values from several filings. The accession number says which one you are looking at.