In the typical submittal scenario, you would create submittals in advance for submissions that you expect to receive outside of a submittal. That is, you expect to receive multiple individual files, not a single submittal file. For information on creating submittals, see Creating a transmittal and Selecting the recipients of a transmittal.
If you receive documents as submissions that are not contained in a submittal file but you still want to associate them with transmitted vault documents, you can do so using implicit submittals. An implicit submittal is a submittal to which you import documents that were submitted outside of a submittal but are related to transmitted documents. The association of the documents to the submittal is implied because the documents were not explicitly included in the submittal.
To import a submission without a submittal:
Note The Status value of the imported submission in the implicit submittal is Checked Out. If you then import the submission into the vault from the submittal, the only available option is Import as revision. You cannot import the submission as a new document because by dropping the submission onto an existing vault document you imply that the dropped document is a revision of the vault document. Proceeding with the import will import the content of the submission as the content of a new revision that is started for you of the document where you dropped the submission.