Okay, I found what I was remembering:
NCE USB Adapter Limitations
The NCE USB Adapter doesn’t support all of the current JMRI features and functions. Some of the restrictions are based on the type of system the USB Adapter is connected to. The USB can’t get information from AIUs, so they can’t be used to get feedback from the layout. The turnout feedback mode MONITORING isn’t available when using a USB, and the Clock functions found in tools are also not available. The USB when connected to a Power Pro system doesn’t support any type of loco programming, and when connected to a SB3 only operation mode (no program track) is available for loco programming. Most functions and features that are not supported by the NCE USB are disabled or grayed out in the JMRI program.
The USB Adapter when connected to a Power Cab or SB3 can only address accessories (turnouts) between 1 and 250. There isn’t a limitation when connected to a Power Pro.
USB Adapter Limitation Summary- All systems:
- No AIU support, No Turnout MONITORING, No Clock functions.
- Power Cab and SB3:
- Can only access accessory addresses between 1 and 250.
- SB3:
- No programming track support, operation mode only.
- Power Pro:
- No loco programming
This is from the JMRI site: http://jmri.sourceforge.net/help/en/html/hardware/nce/NCE.shtml
Not sure what these mean to Power Cab users. I suppose with Power Pro it would be best to use the serial connection built into the command station.