There are some field names missing in the default list of field names when adding a new tag, which are supported by the ID3 tag type. I'd like to know if I can e.g. store 'TIT3' (subtitle), 'TPE3' (conductor), etc.
By default e.g. only 'URL' is included in the list of field names when adding a new tag. Though foobar does support other field names not displayed by default, like 'WWW' as a field name which gets stored in the field code 'WXXX'. Is there a complete mapping list of field names available which are supported by foobar and get mapped to a specific field code?

I also noticed that we should use 'WWW' now to store a tag in the user defined url field code 'WXXX', this used to be 'WWWUSER'.

Current mapping situation:
'WWW' -> 'WXXX' (User defined URL link frame)
'URL' -> 'TXXX' (User defined text information frame)

The old 'WWWUSER' gets split up in 'WWW' and 'USER', displayed in foobar again as 'WWW USER', and is being stored in 'WXXX' with 'USER' as the name. Since the name got changed anyway, why not map URL to 'WXXX' instead? This way 'URL' can be used for each different tag type.

Since I used 'WWWUSER' as a tag the first time, this tag stays in the list of field names. Is there a way to remove this entry? The only thing I can think of would be to alter the 'foobar2000.cfg' file manually. Will it cause any problems if I remove that one line?

Yes, you can.
TIT3 is mapped to SUBTITLE
TPE3 is mapped to CONDUCTOR

Thank you. Do you know of any others not included by default? It would be good to have a complete list in the foobar wiki (help).