This access code makes it possible to prohibit access to the current record for some users.
As a matter of fact, if the field is completed, only the users having this access code with read rights (respectively write rights) can view (respectively modify) the concerned record.
The execution right associated with a user code is processed in a particular way in the case of the import-export templates : if a user has not been granted the execution right, they cannot use the template to import or export data.