b997eee8_CCG___What_s_That_Inside_Joke___In_joke
[Card] The Master (14) - Game Master [Exp.] RD3 [Type] Ref/Pun [In-joke] 
The subtitle refers to role-playing game terminology, since the computer game the card is sourced from is an RPG of sorts. The term is a natural since it contains the word "Master".
Unauthorized Dr. Who CCG - What's That Inside Joke?
In-joke