You can find the answers on your questions below:
1) Every property which is not in the OpenAccess model can be considered as transient.
2) If you are not using OpenAccess Clasic Mapping, the transient attribute is obsolete.
3) When the additional(hand-written) property is not in the OpenAccess model then OpenAccess runtime doesn't know that this property is existing, on the other side, the OpenAccess runtime knows that the transient property is existing, but also knows that the property is not in the mapping.
I hope that helps.
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