...
Reserved slots are used when a Pipeline or validation is executed manually from SnapLogic Designer. If no slots are reserved, then these interactive Pipeline executions—Pipeline executions that run from the Designer for testing or validation purposes—can be blocked by long-running executions. You can adjust this setting based on your experience if the default value is not optimal. To reserve more slots for interactive sessions, set your value lower. If the Snaplex is not being used for interactive Pipeline building, set this value higher.
...