Upon googling, I only find references to python.
The production code is used during the production of a TV show to clearly identify its episodes. It is used instead of the name or the order of the episode within the season because those things can change during production. The production code is only for internal purposes and therefore not susceptible for change.
A production code number, also known as the production code (PC) or episode code is an alphanumeric designation used to uniquely identify episodes within a television series.
I'm not sure what you googled but searching for Doctor Who and "production codes" brought up several answers.
The production code according to TV Tropes Episode Code Numbers are usually used to identify the order in which shows are made and are to be shown. In the case of Doctor Who they only represent the order in which they are made. As they explain:
Doctor Who used an increasing number of letters from Seasons 1-26 (A-Z, then AA/BB/etc. and later 4C, 7D, etc.), then Seasonal Numbering with the revival. A list is here. Note the three codes for The Trial of a Time Lord — that 14-part story is effectively made up of four separate chunks (The Mysterious Planet, Mindwarp, Terror of the Vervoids, and The Ultimate Foe), with the last two produced together as a single block and sharing a code (7C). Also notice that the codes for some Season 30 (Series 4) episodes don't match the broadcast order — this isn't a case of Executive Meddling, but of the show's creators altering the plot as the season was being filmed; they kept the original codes to avoid confusion.