The SysML units define the dimensions used in projects. For example, meter, inch, kilometer, mile, and light_year refer to the concept of length. Therefore, they are all associated to the dimension Length. Every unit must have a SysML valueType to express a given quantity in a standard way so that other quantities having the same unit can be compared. The unit is typically taken from a standard library of units, such as SI or NIST that are part of the SysML profile.
You might define units and value
types for your project from diagrams, or the browser. To define units
in a diagram, select the Unit button
, and
place the new unit into the diagram. To use the browser, expand the TypesPkg,
and highlight Units.
To create extensions
of the UML data types in block definition diagrams, use a ValueType,
as a data type, such as a type for a value property for a block. A
UML data type typically expresses a quantity in a software implementation
type, such as a float or double. In SysML, the valueType expresses
a quantity in a standard unit, such as milliampere. The valueType also
includes a placeholder for the quantity plus the unit. 
In SysML, you can define a valueType without a unit. For example, when the valueType expresses a ratio, the valueType expresses a quantity only.