Isomorphic controls could only be operated by one user. Such controls ostensibly worked only after identifying the allowed user through genetics or other uniquely identifying properties, such as their biological morphic field, of which the name "isomorphic" was derived from.
The TARDIS' controls were said to be flatly isomorphic, (TV: Pyramids of Mars) or at least uniquely operable by the Doctor. (TV: "The Rescue") This fact prevented a shape thief from stealing the TARDIS, even though he had perfectly mimicked the first incarnation of the Doctor. (AUDIO: Mother Russia) However, Sekhmet described isomorphic controls as "a transparent lie, fit only to deceive a child," i.e. a bluff. The Fifth Doctor seemed to agree with this sentiment by simply stating, "Oh, it worked before." (AUDIO: The Bride of Peladon) Indeed, there were several instances of companions (and non-companions, in the case of Utopia) successfully using — or even being actively taught to use — the TARDIS controls. (TV: The Pirate Planet, Castrovalva, Four to Doomsday, Utopia, The Sontaran Stratagem, Journey's End, The Time of Angels, The Pandorica Opens, Journey to the Centre of the TARDIS) The controls were only isomorphic when the Doctor bothered setting them up that way. (PROSE: Original Sin)
Kazran Sardick's machine had isomorphic controls. When Kazran told the Eleventh Doctor this, the Doctor initially said, "Fibber". After Kazran operated the controls when the Doctor was unable to, the Doctor conceded the controls were isomorphic. After the Doctor changed Sardick's personality, they no longer recognised him. The Doctor stated that Kazran's father Elliot, who had set up the isometric controls for him in the original timeline, would not have done so with the altered Kazran. (TV: A Christmas Carol)