SSD for PBP

[z4v4l]

The datasheet listing for components only lists SPI NOR.  Is there a SPI NAND installed on the PBP?  If not, then the hardcoded ROM boot order for the PBP is: SPI NOR, eMMC, SD, USB OTG.  Correct?

NAND is not present, the order is correct.

So to clarify, in order to work, uboot can only be placed on the devices listed in the above ROM code boot order?

Of course! Unless uboot can be split into parts and distributed through several storages with one part loading another. but anyway, that first part (let's call it SRAM part - because it gets loaded into SRAM) can be only on devices from that list (obviously). and also, again, USB is not as USB mass storage, but as USB OTG, connected to another computer (host) and exchanging data/commands through a custom protocol (you need to use rockchip tools on the host). this option is not even for uboot, but only for its early components (basically for the rockchip blobs for memory initialzation), so it is quite useless for the general public (unless they are ready to try to understand rockchip's wiki and tools ).
for the sake of completeness, here is a short excerpt of the block scheme on what ROM code does with that mask ROM mode (USB OTG).

Initialize USB port.

1.Wait request for download DDR image code
2.Download DDR image code to internal SRAM
3.Run DDR image code
4.Wait request for download loader image code
5.Download loader image code to DDR
6.Run loader image

This leads me to believe that "At this time, the boot order for the custom uboot (on eMMC) on the default Debian + MATE build is: SD, USB 2.0, eMMC" means the SPI firmware is flashed to point to uboot on the eMMC.  Is that correct?  Then uboot on the eMMC looks for an OS on the SD, then USB 2.0, then eMMC?

is SPI flashed? with what? it's not. when it is, it's uboot there, the firmware is uboot (in all possible components - rockchip blobs, SPL/TPL, ATF etc), there is nothing else over there. when SPI is not flashed, uboot is on eMMC and gets loaded from there. uboot looks for the OS, based on how its environment variables and scripts are configured (this is what depends on the builds and what is changeable by users in the scope of uboot possibilities).