You're seeing just the function for_update, go back to Ash.Changeset module for more information.
Link to this function

for_update(initial, action, params \\ %{}, opts \\ [])

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Constructs a changeset for a given update action, and validates it.

Anything that is modified prior to for_update/4 is validated against the rules of the action, while anything after it is not.

See for_create/4 for more information