pfSense

Steve Wheeler wrote in #note-1:

[...]
That will than send traffic tagged with vlan id 0 and the pcp tag set.

It is surprising to me that a system is designed to set a value to zero by setting another parameter and having as a side effect the un-initialised value of a parameter not set is what is desired outcome.

To explain my confusion
802.1Q tag format consist of
a 16 bit Tag protocol identifier (TPID) field set to 0x8100
a 16 bit Tag control information (TCI) field consisting of
... a 03 bit Priority code point (PCP)
... a 01 bit Drop eligible indicator (DEI)
... a 12 bit VLAN identifier (VID)

A case could be made to add the ability to set the 16 bit TCI directly
I can understand the value in being able to set the VID to 0 using the existing GUI (perhaps with a warning that this is not valid only in special circumstances.
I don't understand why setting the 3 bit PCP field should be the recommended way of setting the 12bit VID being set to zero.

Clearly I'm not an expert in network protocols so perhaps it is perfectly logical. It just seems a counter intuitive system design approach to me.