Dear Stephen Warren,
Again, this is confusing two different kinds of partitions.
There are HW-level partitions/regions/areas within the eMMC HW itself. You need to send commands to the eMMC device to select whether read/write commands act on the boot0/boot1/general*/user HW partition.
This will be done via "mmc boot [dev] [HW partition (boot0/1/user]" command (from u-boot prompt). This command is not yet implemented at u-boot (at least for Trats development board). After its implementation it will be used as a helper function for dfu.
As a result the access to those partition will be done via proper DFU's alt settings: mmc-boot0 mmc-boot1 etc.
There are (or can be) SW-level partitions within any/all of those HW partitions. This is the level at which an MBR/GPT partition would exist.
With DFU, I'd expect an alt setting for each of the HW partitions at least.
And this is my goal. Now only the "user" HW partition is supported. After "mmc boot ..." command implementation support for other partitions will be added for DFU as well.
With UMS, I'd expect a device to appear for each of the HW partitions. (these may show up as say /dev/sdb for boot0, /dev/sdc for boot1, /dev/sdd for the user area).
Frankly speaking I've thought of providing access only to user HW partition for initial UMS implementation (the UMS v1 implementation). However access to all HW partitions from UMS is worth consideration.
If an MBR/GPT is present within any of those, Linux may then create a device for each SW partition, so e.g. /dev/sdd1, /dev/sdd2, etc.
This is the functionality which now UMS (v1) provides.