Describes the usage of a DataField within a FormLayout.
Enumerator:
eVarUsage_NotUsed
The DataField is defined within the Project but is not used in the FormLayout.
eVarUsage_Normal
A normal DataField. The DataField is in use by the FormLayout, its value is not modified. The DataField is just used as part of expressions or directly in text-, bar-code or picture elements. Therefore the value for this DataField may be provided by an external application or by the user.
eVarUsage_Calculated
The DataField is computed automatically. The DataField is in use by the FormLayout and its value is modified. Usually this is done via pre- or post-expressions. Therefore the value for this DataField may not be provided by an external application or by the user.
eVarUsage_Parameter
This is a writable system-DataField used by the FormLayout. The value of this system-DataField may be provided by an external application or by the user (e.g. tfDocumentName).
