# Message PT, Trgt, and Src

The **PT** (Priority / Type), **Trgt** (Target), and **Src** (Source) fields define the first 3 bytes of a message in the Messages Editor for these networks:

* J1850 VPW with 3 byte header\*
* ISO9141 / Keyword
* J1850 PWM

\*Note: Use the Tools/Options/Vehicle Networks tab, [J1850 VPW Mode](https://docs.intrepidcs.com/vspy-3-documentation/3.9.12/vehicle-spy-main-menus/main-menu-tools/tools-options/options-spy-networks-tab/spy-networks-tab-j1850-vpw-mode) selector to switch between the 1 byte (Chrysler) and 3 byte (GM Class 2) headers. If a 1 byte header is selected, then the first 3 bytes will be ID, B1, and B2.