Allied Telesis AT-S63 - Manual (Page 606)

Chapter 24: Port-based and Tagged VLANs
606 Section VI: Virtual LANs
Note
Untagged ports of a 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 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.