In order to lock/unlock a test suite, click on the checkbox to the left of the test suite name and then click on Lock/Unlock depending on the current State. If a test suite is locked, no other user can modify it unless that user is the owner of it. However, if a test suite is unlocked, any user can edit it. Only the owner of the test suite and a user with admin role can perform locking/unlocking.
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The State column on the Test Library page will show you who locked the test suite to make it easy for coordination. In the above image, the test suite was locked by the user.
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