The Client argument is documented in the Developers Guide. It is a native GroupWise Client object. Here’s some information from the Developers Guide, which also contains a link to online documentation:
Client: This argument represents the state of the GroupWise client at the time your applet was executed. It is the same as the GroupWise C3PO ClientState Object. Your applet should not assume a specific Client object is in a particular state. Rather, the ClientName, MajorVersion, and MinorVersion properties should be checked to identify the appropriate state of the object. Full documentation of the Client argument can be found at: http://developer.novell.com/ndk/doc/gwc3po/gwc3penu/data/hye3ji8g.html
All other objects are documented in the Language Guide. In terms of the message store access object you mention, search for ‘composingitem’ and ‘account’ in the index.
I hope this helps.