TACACS+ user roles and the priv-level attribute

The priv-lvl attribute defined on the TACACS+ server, results in the following user role assignment on the switch:
priv-lvl User role assigned Description
Not Set None (authentication error) priv-lvl not set, so the user is denied access.
1 operators Map the user to the operators role as TACACS priv-lvl 1.
2 to 14 None (authentication error) User roles 2 to 14 are not available. Users with these roles are denied access.
15 administrators Map the user to administrators role as TACACS priv-lvl 15.
19 audittors Map the user to auditors role as TACACS priv-lvl 19.
<1, or 16, 17, or 18, or >19 None (authentication error) Invalid priv-lvl value, so the user is denied access.