Is every employee a user?

This may not necessarily be true. Every employee of your company may not use Vtiger CRM.  The User field in the Employees module is used to identify employees who are CRM users.

When you create a user in Vtiger CRM, an employee record is automatically created for the user, and the User field is enabled.

However, when you create an employee, no user record is created. Thus, the User field is not enabled.

To learn more about Employees in Vtiger CRM, click here.
To learn more about Users in Vtiger CRM, click here.