When you first sign up with Manage My Vessel you are given an account, which is your portal that you have been setting up throughout this guide. This is called your Fleet Account.
Whenever you add a vessel, or similar, to the system this is called a Sub Account. The sub account is where the day to day interaction will occur between the vessel and the company and is where all of the records are stored for each particular vessel.
Sub accounts are kept separate and crew can only log into a single sub account with the username and password provided.