I don't believe that is correct. Optional will allow either untagged or tagged packets, but you still need to tell the switch what VLANs are on which port. At least that is my understanding (and my observation for ports that I have set to optional).
I just did a test. I have WiFi access points that use an untagged LAN for management, and up to four tagged VLANs for different SSIDs. One of those is on port 3 of my data cabinet switch (a CSS326-24G-2S+RM with 2.12 firmware). Therefore, on the VLAN page, the VLAN mode is set to "Optional", the VLAN Receive is set to "Any", and the Default VLAN ID is set to the VLAN that is used for AP management. I started a continuous ping to the AP's IP for one of the SSID (on VLAN 103) from my desktop computer. The pings were working perfectly. I then on the VLANs page, removed the Members checkbox for VLAN 103 for port 3 of the switch. The pings started failing. I then replaced the checkbox for VLAN 103 on port 3 and almost immediately the pings started working again.
This matches my expectation. You have to specify what VLANs are in use.
RB750Gr3, RB750r2, CRS326-24G-2S (in SwitchOS), CSS326-24G-2S, CSS106-5G-1S, RB260GS
Not sure if I beat them in submission, or they beat me into submission
Warning: I know enough to be dangerous...