<units_system>

Unit system definition
The field used to choose the unit system is special. It has the attribute units_system_definition="1", it does not contain any "v" attribute, and it contains a unique dependency related to the unit fields.

n unit name

pn printed name

unit_mesh_definition The field used to choose the mesh unit is also special. It has the attribute unit_mesh_definition="1", and it does not contain any "v" attribute or dependencies.

unit_definition The fields used to choose the default units in the GUI are also special. They contain the attribute called unit_definition="magnitude" being magnitude the name 'n' to be used in that field. It is important to note that these kind of fields does not contain dependencies.

Here's an example of node type "container" in the .spd file, which allows to choose the geometry units and the general units, as follows,

<container n="units" pn="Units" icon="units-16" help="Units definition">
      <value n="units_mesh" pn="Geometry units" unit_mesh_definition="1" icon="units-16"/>
      <value n="units_system" pn="Unit system" units_system_definition="1" icon="units-16" state="hidden">
        <dependencies node="//*[@unit_definition or @unit_mesh_definition='1']" att1="change_units_system" v1="{@v}"/> 
      </value>
      <container n="basic_units" pn="General units" icon="units-16" state="normal">
        <value n="units_length" pn="Length" unit_definition="L"/>
        <value n="units_mass" pn="Mass" unit_definition="M"/>
        <value n="units_force" pn="Force" unit_definition="F"/>
        <value n="units_pressure" pn="Pressure" unit_definition="P"/>
        <value n="units_temperature" pn="Temperature" unit_definition="Temp"/>        
        <value n="units_time" pn="Time" unit_definition="T"/>
       </container>
</container>

Example of window to specify the selection of units

International and Imperial systems
There are two primary systems used to define units: the international and imperial systems, as seen below. The international system of units is the modern standardized form of the metric system. It sets standard measurements and conversions, and is the most commonly and universally accepted system of units. The imperial system, also known as British Imperial, is the system of units first defined in the British Weights and Measures Act of 1824, which was later refined and reduced.

Unit system	n
Int. system (SI)	SI
Imperial system	imperial

The fields of type <unit> can contain the attribute called units_system, as follows,

units_system - There are two systems used to define units, the possible values are:

SI - International system
imperial - Imperial system

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