eDiaries are a function of patient reporting (ePRO) which allows collection of data from patients at regular intervals (e.g. Weekly, Monthly, etc).
Defining and editing patient diary design is done within the Study Configuration menu:
Setting up eDiary on TrialKit is the same as the way it's done in Clinical Studio. Read here for details on how that is done.
For collection of patient surveys within a visit schedule, read here.