8 pages of results (with the default setting of 25 per page)for "subpanel" appear for the time period of: "Newer than 3 years", and "Older than 1 day".
The search engine only searches for a 3 year period at a time, so if you were to enter "Newer than 6 years" and "Older than 3 years" the next result period would appear.
(Googling "subpanel" brings up a lot of DIY sites)
(If you enter "subpanel" on a word search of an NEC CD you will get 0 results. The word appears nowhere in the text of the NEC. John V is correct in that there is no "subpanel", per se, manufactured)