Hello,
Starting in R2019a, the 'ModelReferenceRTWTargetOnly' option for "slbuild" is no longer recommended due to clarity in the name. Instead, 'ModelReferenceCoderTargetOnly' is recommended. See the image below from this documentation.
To understand 'ModelReferenceRTWTargetOnly', see the "slbuild" documentation for previous release (R2018b). The difference between 'ModelReferenceRTWTarget' and 'ModelReferenceRTWTargetOnly' is the 'model reference simulation target', that is, whether or not a MEX file is created for each model reference. 'ModelReferenceRTWTargetOnly' does not create a MEX file.
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'ModelReferenceRTWTarget'
Builds a model reference Simulink Coder target and the corresponding model reference simulation target
'ModelReferenceRTWTargetOnly'
Builds only a model reference Simulink Coder target
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When you generate code for a referenced model hierarchy, the code generator produces a standalone executable for the top model and a library module called a model reference target(Simulink Coder target) for each referenced model. When the code executes, the top model executable invokes the model reference targets to compute the referenced model output. Model reference targets are sometimes called Simulink® Coder™ targets.(Generate Code for Model Reference Hierarchy)
Starting in R2020b, the buildSpec argument values 'ModelReferenceCoderTarget' and 'ModelReferenceCoderTargetOnly' have the same behavior. Therefore, there is no need to distinguish between the two options but 'ModelReferenceCoderTarget' is recommended.
