CRS location

Not much is said about where the CRS is supposed to appear; the introduction says

The TFCat object contains a list of fields defining the properties listed in the Features, and a coordinate reference system (CRS) describing the temporal and spectral axes of the geometry data.

but I don't see any further reference in the normative text.

Some content should be added saying where a CRS object may/must appear, whether more than one is permitted, and if applicable what happens in the case that fewer or more than exactly one is present. Maybe copy text from the https://gist.github.com/sgillies/1233327#file-geojson-spec-1-0-L227 draft you referenced earlier.