How can we select which documents are sent to training?
We can manage which documents will go (and which won't) to the training stage via scripting.
The IDocument interface has the IsUsedForTraining property which can be accessed and utilized via script rules in the document definition or script stage in the workflow.
For example, we can check the value of the fields or the registration parameters value and send the documents to the training by setting the IsUsedForTraining value to True. In the same way, we can set this property to False which means that this document won't be used for training (it will go to the training stage, but Training will be skipped for it and it won't be added to the training batch).
This approach can be considered as the solution at the current moment because the Entry Conditions for the Training stage have a different design than other stages and for now there are no plans on introducing the Entry Conditions feature for the Training stage.