The inborn special skills of the Alter Egos.
They were the results of illegal modification. id_es skills are the Alter Egos’ existing abilities that are changed or reinforced to be stronger.
Most of them are skills that normal Servants do not possess.
They are skills that reflect the mind or emotional embodiment of the Alter Ego, rather than being based on their physical ability (body).
Besides Passionlip’s Masochistic Constitution, Meltlilith’s Sadistic Constitution,
there seems to be other types of id_es, including Infantile Regression, Bondage Paraphilia, and Cognate Aversion.

Fate/EXTRA Material: Encyclopedia of Fate/EXTRA