ID3v2 has special TXXX (user defined) fields, which allow you to have fields of any name. In Mp3tag, if a name doesn't map to one of its recognized four-letter frame names, then Mp3tag will create a TXXX frame with that description. So you can write things like 'Catalog #' or "Catalog Number'. This is commonly how ReplayGain fields are written to ID3v2 tags - TXXX frames with descriptions of REPLAYGAIN_TRACK_GAIN, REPLAYGAIN_ALBUM_GAIN, etc..
You can read the ID3v2.3 and 2.4 specs here, but they've been extended quite a bit with de facto standards by applications, mostly iTunes. There's no mention of a frame to store catalog numbers. Googling around a bit, I can't find anything suggesting a de facto standard, either. It seems that most often it's written into the comment (COMM) field.