This is not a permanent state such as, for example IsReadOnlyFile . Value of this property changes dynamically multiple times within the life-time of an open document. Regardless of the mode a document is opened with, the model can only be modified inside an open transaction. Furthermore, even with a transaction open, the model is not always modifiable. Though this is rather a rare situation, it can happen, most likely during model regeneration, failure processing, or some events. An attempt to modify a non-modifiable document will result in throwing a ModificationOutsideTransactionException. See also IsReadOnly