So for the 880, it sounds like the device priority order is: Master Activity Device, Sound device, Display Device. The Master Activity Device is based on the Activity Type. Examples: Tuner Device for a Watch TV type, Player device for a Play DVD type, and so on. Harmony changed the device priority order on the 880 when they introduced it without the Sound and Picture buttons. I don't know if this new priority order has propagated to the 6xx series since I have not updated my 659/680 in a very long time. I am still waiting for them to fix the ASSERTION bug on the new "Set Up Favorite Channels" ported from the 880.

In the "Custom Button Label Area" (LCD), a command name can only be used once per column (slot). This limitation applies only within the "Custom Button Label Area". Workaround: Teach the command under a slightly different name.

To find dummy commands, just try them all in device mode. You can also teach some from the OEM remote of an obsolete/unused device.