ash v1.51.2
ash

Ash.Type.handle_change

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

handle_change(type, old_value, new_value, constraints)

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Process the old casted values alongside the new casted values.

This is leveraged by embedded types to know if something is being updated or destroyed. This is not called on creates.