Android firmware image components explanation

Question

Consider unpacking an android firmware image made for a Media box using for example the C code in this post or by 'Amlogic customization tool'.

When I use the compiled C code with an android 7.1 image, I get these files:

_aml_dtb.PARTITION
aml_sdc_burn.ini
aml_sdc_burn.UBOOT
boot.PARTITION
bootloader.PARTITION
DDR.USB
logo.PARTITION
manifest.xml
mesonl.dtb
platform.conf
recovery.PARTITION
system.PARTITION
UBOOT.USB

Excluding some minor config files here, I know each of these images are a partition in my device's eMMC. But there is some questions about these files:

1- I know bootloader.PARTITION is the same as u-boot.bin in out folder of my android sources directory after compilation. But what are UBOOT.USB and aml_sdc_burn.UBOOT files? I guess one is used when burning image via USB tool and one for burning via SD card. But I do not have any idea what these files contain and what is the relation regarding U-boot.bin?

2- What is DDR.USB file?