SysML: Whether you use a Block, ValueType, or Signal to represent something that flows (and can be applied to an ItemFlow) depends on what you want to achieve. If you want to indicate something structured with value properties with Units use a Block.

You can use a structured ValueType, but any quantity properties within it typed by ValueTypes won't be able to display any units, because a ValueType symbol does not have a values compartment. In this case you should probably use a structured Block with value properties.

One reason modellers tend to go for ValueType first is that if you choose a Block and use it to type a part property to be used for an ItemFlow::itemProperty, that property will list in the parts compartment, although conceptually it is just a "packet". This tip can help there:

Webel Best Practice: SysML-1.6/1.7: You can introduce a custom (user-defined) stereotype keyword «item» to indicate Properties that are to be used as "packets" for ItemFlows on Connectors. (SysMLv2 will have a formal way of treating such items/packets.)

Signal is a nice initial choice if you want to distinguish purely digital signal flow (such as a sensed TemperatureReading) from the flow of quantities involved in physical interactions. Use of Signal is fine as a conveyed Classifier for basic InformationFlow style ItemFlows, however, it's not well suited to a finer grained ItemFlow::itemProperty.

Beware that:

SysMLv1: MagicDraw/Cameo: GOTCHA: If you DRAG a Signal onto a Block symbol it will create a Reception. One way to create a Property typed by a Signal is to use a composite Association. Or just create an untyped Property then type it by the Signal.

SysML: Flows: If you use a Signal to type the itemProperty of an ItemFlow on a Connector, care must be take in how you create the Property or you might get an unwanted signal Reception on the owning context Block.

If you are using SysML Extension for Physical Interaction and Signal Flow Simulation (SysPhS) for physical interactions you MUST use a Block:

SysML+SysPhS: Flows: If you intend to use a Classifier to type the itemProperty of an ItemFlow on a Connector for a physical interaction you MUST use a Block (not a ValueType or Signal) so your can extend ConservedQuantityKind.

Relates to

Package Diagram overview of a simple domestic air conditioner with an inverter

Overview of ValueTypes and Units

Related notes