::Richard,
The ideas of a ‘worker’ and a ‘user’ record are separate things within D365FO.
A user record is required for someone that is logging into D365FO, a user record is also where role assignments occur which will dictate what access this user has.
A worker record is used within D365FO is used for workflow approvals as well as certain external portals throughout the system (production floor execution mobile app, warehouse mobile app, etc). These individuals normally interact within D365FO through a mobile device and have extremely limited functionality they can perform, therefore workers do not require a license within D365FO.
The reason these are separate ideas is because you have have someone that needs to be a worker within D365FO but does not need a user record (saving on licensing costs). However you can associate a user and a worker record (if this is possible) via the Person field on the user record.