Allied Telesis AT-S63 - Manual (Page 614)

Chapter 25: Port-based and Tagged VLANs
614 Section VI: VLANs
Note
When you create a new VLAN, ports designated as untagged ports
of the new VLAN are automatically removed from their current
untagged VLAN assignment. For example, if you are creating a new
VLAN on a switch that contains only the Default_VLAN, the ports
that you specify as untagged ports of the new VLAN are
automatically removed from the Default_VLAN.
Note
Tagged ports are not removed from any current VLAN assignments
because tagged ports can belong to more than one VLAN at a time.