For fields in EUCTR (protocol- and results-related information),
see also the register's documentation at
https://eudract.ema.europa.eu/.
For fields in CTGOV (protocol-related information), see also
the register's definitions at
https://prsinfo.clinicaltrials.gov/definitions.html.
Note: Generating a list of fields with this function may take
some time, and may involve running a mapreduce function is
run on the server. If the user is not not authorised to run
such a function on the (local or remote) server,
random documents are sampled to generate a list of fields.