yaz-client is a line-mode Z39.50/SRU client. It supports a fair amount of the functionality of the Z39.50v3 standard. Its primary purpose is to exercise the package, and verify that the protocol works OK. For the same reason some commands offers more functionality than others. Commands that exercises common Z39.50 services such as search and present have more features than less common supported services, such as Extended Services (ItemOrder, ItemUpdate,..).