Hi Luo,
as mentioned by Riaan you can define for each workflow / CR step if changes to the object are allowed or not.
Talking about the predefined workflows and CRs provided by SAP, consider the same as templates! It just an example what can be done, but surely needs some fine tuning according to a specific customer project needs.
You're right that the pre-defined example denies changes in the "to be revised" step. The reason for that is the target scenario for this predefined workflow / CR type. The requestor should be able to enter anything. In the second step, the master data expert decides whether to finalize or to reject the request. If the request is OK, it's the task of the expert to fix invalid information or additionally maintain empty attribues. If the request is not OK, the expert rejects the CR giving an explanation why this decision has been taken. The "to be revised" status is more or less used for informing the requestor about the rejection.
Best regards
Michael