I think the author is implying that T transforms the data (implicitly) to a target schema. So, if T comes at the end (at query time) you are in effect schematic

I think the author is implying that T transforms the data (implicitly) to a target schema. So, if T comes at the end (at query time) you are in effect schematics (or using the schema implied in the query). If T comes before L you are mapping to a target schema before L.