Hypothetical ramble, given to understand. I'm aware of other viewpoints arguing for their respective sides (Enton: Kagutsuchi vs. Tsukuyomi).
Offering greater clairvoyance to its keeper, the Sharingan begets an atypical Ninjutsu possessing properties of a Genjutsu birth from the Mangekyou; Tsukuyomi. By eliminating restrictions, Itachi was able to fashion a world where limitations are nothing but mere fabrications. In such a world, the user is able to remove or incorporate physical advents: time, gravity, and space, are all controlled by the user.
Noticeably; Enton: Kagutsuchi or Blaze Release has similar properties where the user exerts that same control over the pinnacle of Katon-based Ninjutsu; Amaterasu. By administrating Keitai Henka to Amaterasu, the user is able then able to manipulate its existence; determining the purpose of the technique.
Being siblings, Itachi and Sasuke possess similar abilities birth from their legacy and prodigious aptitudes.
Itachi labels his secondary Mangekyou technique as Tsukuyomi, a Ninjutsu that after being manipulated reconstructs as a Genjutsu technique, attributable to Itachi’s volition: comparable to Sasuke’s labeled technique. Enton: Kagutsuchi.
I put forth the proposal that the secondary ocular power behind both Itachi and Sasuke is the ability to manipulate any jutsu—which may heighten by acquiring a higher Dōjutsu, enabling higher forms of that manipulation. Principally constructing realms of phenomenal creation, that is capable of washing away pollution; Mangekyou blindness and the endowment of new abilities.