ash v1.51.2
ash

Ash.Type.prepare_change

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

prepare_change(type, old_value, new_value, constraints)

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Process the old casted values alongside the new uncasted values.

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