Jag är inte säker på vad du googled men letar efter Doctor Who och "production codes" har tagit upp flera svar.
Produktkoden enligt TV Tropes Episode Code Numbers används vanligtvis för att identifiera den ordning i vilken visar är gjorda och ska visas. I fallet med Doctor Who representerar de endast den ordning i vilken de är gjorda.
Som de förklarar:
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.