The dtype is a SYSTEM variable and defined in the SyBase database. This variable is not visible through GUI but can be used. This variable is mapped to the value of device in the parser. You can find the value of the dtype in <devicename>.ini file in the parser folder under /etc/devices/<devicename>. The ini file has an entry line "DeviceType=<somenumber>. This is what is used in reports.
Eventtype variable is defined in the GUI and can be found under Overview -> System Configuration -> Messages -> Manage Variables.
Hope I answer your question.