Question: What are the document fields available on the Dialog platform and how do they differ from each other?
Answer: There are actually 3 types of document field that are available on the Dialog platform:
a/ Rtype (or rtype.exact) This will search the Information Provider (IP) supplied document type field.
b/ At (or at.exact) This will search the PQ normalised document type field.
c/ Dtype (no dtype.exact version available) This will search either the Rtype or At document field. As this is searching two different fields, that is why dtype.exact will not work.
When searching the Advanced form, this will contain IP-supplied values in most cases - and these should match the values displayed in the Document type field in the record itself. The underlying search for these uses mnemonic RTYPE (or RTYPE.EXACT) - you should see this when you export your searches.
The following exception will occur. In exceptional cases, the PQ-normalised values can be listed on the Advanced Search page - but only if the IP has supplied more values (ca. 100) than can accommodated in a complex limiter. The underlying search for these uses mnemonic AT (or AT.EXACT) - you should see this when you export your searches.
This latter exception will appear for databases such as Gale Group Promt. Databases such as Biosis, Embase, Medline will have Rtype for document type field.
Also, because a & b represent fields with no structure, EXACT searching will work.
The above is why rtype, rtype.exact and dtype will retrieve the same number of records as they are searching the same data within the biomedical databases.