Command Paper is the collective name given to different types of papers prepared by the Government and presented to Parliament with the words "presented to Parliament by command of His/Her Majesty". The following documents are all types of Command Paper: Treaties, some White papers, some Green Papers, some Government replies to Select Committee reports, reports of Royal Commissions, reports of some major committees of inquiry, annual reports and statistics of certain bodies (judicial, penal, immigration).
Further information on this subject can be found from the following links.
About Parliament: Government publications
Command Papers - Official Documents