Revision history [back]

This tutorial provides a short description. Another short discussion is here. To summarize, the level is just a bit mask that allows you to group reconfigurable parameters for your own use in the service callback. For posterity, I found this tidbit (from the second link) useful for understanding:

I had this discussion with Blaise Gassend a year or so ago. He said the level parameter is just a bit mask. It can be used to classify sets of parameters in any way that is convenient to the package defining the server. The bits for all parameter classes that have changed will be ORed together for the service callback.

One useful classification scheme is suggested by the driver_base states:

1. Parameters that can only be changed while the device is closed.
2. Parameters that can be changed while the device is not streaming.
3. Parameters that can be changed while the device is open and streaming.

But, servers are free to classify their parameters any way they like.