MDIS has standardised on the following definitions for Mandatory and Optional items. In all cases (Mandatory or Optional), if the item is available, the functionality described by the definition of the item must be correct and verifiable.
Objects specified as Mandatory will be required in all Objects and cannot be deleted. In OPC terms, a Mandatory item must exist on every Node of the NodeClass, for example if an MDISValveObjectType defines a Mandatory item Position then every instance of the MDISValveObjectType must have an item Position available.
Objects have functionality that may or may not be included; if they are included the OPC Client will know how to handle them. In OPC terms, an Optional item may or may not exist on every Node of the NodeClass, for example if an MDISValveObjectType defines an Optional item OpenTimeDuration then some instances of the MDISValveObjectType may contain an item OpenTimeDuration, but other instances may not. Clients are required to be able to handle the case where the item does not exist.