Ptypes are programmer-defined strings that name tool kinds. (You can roughly translate ptype as process type.) Each ptype can be associated with a set of patterns that describe the messages in which that particular ptype is interested and a string for the ToolTalk service to invoke when an instance of that ptype needs to be started.
The main purpose of ptypes is to allow tools to express interest in messages even when no instance of the tool is actually running in the scope in which the message is sent. If a tool is able to perform a message's requested operation, or wants to be notified when a particular message is sent, it indicates this instruction in its ptype and ToolTalk will start the tool when necessary. Since the ptypes database can also be modified by the system administrator or user, the mechanism allows the site's or user's favorite tool be designated as the tool to handle a particular message.