To use a VM lab, go the "Workspace section" of your part setting and select "VM lab" as the "Lab Type"
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In the "Lab" section, all the labs that were configured by the org admin should be available for selection. Select the lab from the list - 

Under the "Course Parameters" section of the Course Settings, the two applicable parameters for VM labs are - 

This specifies the time for which the lab is launched. A lab can be ended earlier by a user by explicitly selecting "End Lab". Also, when the learner selects "Start Lab" in the middle of a session, the session is extended to the full value of "Lab Session Length" value.

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