If you are using RTC's Pessimistic locking feature, it doesn't work for third party editors like Microsoft Office.
Scenario
- Add Microsoft Office extensions say .docx or .xlsx in Auto-lock files option of Streams.
- Now open the Word document in Eclipse and write something in it. No popup will appear to lock the Word Document as for e.g. .txt file.
- After editing the Word document, close the file to save the changes and the file will be saved although it is in read only mode.
For the above issue, a defect has been identified with third party editors. Below are links to the related defects:
https://jazz.net/jazz/resource/itemName/com.ibm.team.workitem.WorkItem/376941
https://jazz.net/jazz/resource/itemName/com.ibm.team.workitem.WorkItem/352603
Workaround
As the defect is with third party editor, hence no fix is available for RTC. However, you can use the below workaround to make sure that changes are not saved to a read-only Microsoft Office file.
--> Open any Microsoft Office extension (.docx) file using "Open with" option in external editor, the file remains in read-only mode and saving change in the files gives an option to do a "Save As", rather than updating the original file.
To summarize, the read-only doesn't work if the file is edited with Eclipse embedded editor, but the same works if the files is opened using an External editor.
You can also refer the below link for more details:
https://jazz.net/wiki/bin/view/Main/PessimisticLocking
Tags: 
pessimistic
rational-team-concert
pessemistic-locking
auto-lock
locking
rtc
iot-support
ms-offic
ibm-iot