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.

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Note that your choices are restricted to ValueType, Block, or Signal by:

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:

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, because as soon as you create a Property typed by a Signal you'll end up with it being listed under the Signal Receptions compartment, which is confusing and probably not what you want.

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

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