private Object getPrincipalFromClassName() {
        Object principal = null;

        try {
            Class<?> cls = Class.forName(type);
            PrincipalCollection principals = getSubject().getPrincipals();
            if (principals != null) {
                principal = principals.oneByType(cls);
            }
        } catch (ClassNotFoundException e) {
            if (log.isErrorEnabled()) {
                log.error("Unable to find class for name [" + type + "]");
            }
        } catch (Exception e) {
            if (log.isErrorEnabled()) {
                log.error("Unknown error while getting principal for type [" + type + "]: " + e.getMessage(), e);
            }
        }
        return principal;
    }
The default value that should be displayed if the user is not authenticated, or no principal is found.
support/jakarta-ee/src/main/java/org/apache/shiro/ee/faces/tags/PrincipalTag.java