Several categories of persons are subject to immigration control, including British Commonwealth citizens who maintained close ties to Britain through their status as British subjects, Commonwealth citizens without close ties to Britain, and United Kingdom passport holders who are not exempt from immigration controls applicable to other nationalities. While Great Britain's island geography makes it easy to effect control at ports of entry, the large number of passengers actually arriving at those ports place enormous pressures on immigration control systems. Immigration law enforcement deals with three main categories of immigration offender: the overstayer, those who work in breach of their landing conditions, and illegal entrants. Economic reasons motivate many of these illegal entrants. Immigration intelligence is gathered mainly from immigration officers working at ports; it has been especially useful in the area of forgery detection. The British immigration service gives courses in forgery detection to other national services and is working to devise and develop new detection technologies. Although the 1992 open borders will allow citizens of European Community countries to enter Britain freely, immigration officers will continue to inspect all documents to determine that holders are indeed E.C. nationals.