There’s always been 3, no matter what sub-area of the library. Your training, validation, and test. Training and validation are used during training, the test or test_dl is used for inference only. It applies anything done to your validation set to some new dataset.
Z - you cover the standard fastai tabular as well as xgboost and random forests. Do you ever cover exporting trained embeddings to the tree methods? Could always export the embeddings and recreate the full data layer outside fastai, but seems like there should be a way to hijack the fastai feed into a prepped array for xgboost training.