An Account is the element in the model that links trading activity to the users undertaking that activity. All Accounts are "owned" by a Firm_ and it against one of these Accounts that Holding_ records are managed in the system.
This is a reference to the ExternalAccount table. It is keyed by the ExternalAccount column in this table to the Id column in the ExternalAccount table.
UpdateNumber
Integer (unsigned)
8
A number that records every change that happens to a record. It is primarily used to avoid write after read problems when an intermediate write has changed the originally read record.