Confusing while upgrading LEDE, All files explained

I follow this link frequntly to check updates
http://downloads.lede-project.org/releases/17.01.4/targets/apm821xx/nand/

Now there is 4 file which is related to my device, can anyone explain, what are the files, I have guessed 2 of the following, kindly help me identify correctly

WNDR4700-initramfs-kernel.bin - ???
WNDR4700-squashfs-factory.img - Factory ROM to LEDE converter
WNDR4700-squashfs-kernel.dtb - ???
WNDR4700-squashfs-sysupgrade.tar - upgrading from LEDE to LEDE

I have found script to download latest update automatically
wget -r -l1 --no-parent -A "lede-*-apm821xx-nand-WNDR4700-squashfs-sysupgrade.tar" http://downloads.lede-project.org/releases/17.01.4/targets/apm821xx/nand/

The commit message explains the first, second and fourth file.
https://github.com/lede-project/source/commit/9e0fd1b52ad1f805a308bf6a5a13236f352fd962

The kernel.dtb can be used to manually update the compiled Device Tree binary.

This was added in this patch:

https://github.com/lede-project/source/commit/49856a4bb5818146d9cf951cb65a0903d3b38fd4

Note: Netgear decided it was a good idea to put the DTB straight on the NAND. This is not a good idea. There is no wear-leveling algorithm for the blocks, so updating the DTB should only be done, if there was a meaningful change to the .dtb. If the block that the dtb is stored in fails (due to it wearing out), you would have to repartition the device, tell u-boot about it and make custom images.