If you don't specify a value for the Mandatory property, PowerShell considers the input parameter optional by default. If you set this to False, a value is optional.
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If you set this to True, a value must be provided when the runbook is started. Boolean value specifying if the parameter requires a value. This name must be unique within the runbook, must start with a letter, and can contain only letters, numbers, or underscore characters. The data type expected for the parameter value. PowerShell and PowerShell Workflow runbooks in Azure Automation support input parameters that are defined through the following properties. Configure input parameters in PowerShell runbooks You can also start one as a child runbook that is called inline in another runbook. You can start a runbook from the Azure portal, a web service, or PowerShell. You assign values to the input parameters for a runbook when you start it. Input parameters can be mandatory or optional, and you can use default values for optional parameters. A runbook can have multiple parameters with different data types, or no parameters at all. You can configure input parameters for PowerShell, PowerShell Workflow, graphical, and Python runbooks. This article describes the configuration and use of input parameters in your runbooks. These parameters allow runbook actions to be targeted for specific scenarios and environments. Runbook input parameters increase the flexibility of a runbook by allowing data to be passed to it when it's started.