|0x4||0x4||This u8 must be zero|
|0x8||0x100||RSA-2048 signature over the 0x10-byte data at 0x108|
|0x110||0x10||AES engine keyY for 3 keyslots (High u64 is unsigned)|
This keyY is the console-unique portion of the 3 keyslots used for everything stored under sdmc/Nintendo 3DS/<ID0>/<ID1> and nand/data/<ID0>. For SD this is used for encryption and AES MACs, however for NAND this is only used for AES MACs. This file is transferred to the destination 3DS during a System Transfer. The movable.sed keyY high u64 is updated on the source 3DS during a System Transfer, and when doing a system format with System Settings.
Movable.sed always exists on retail(written to NAND at the factory), however if this file doesn't exist the system will fall-back to using a console-unique keyY already in memory, with the last 8-bytes being loaded from the 8-bytes following that u64. It's unknown whether movable.sed exists on development units.
The keyY is hashed with SHA256, the first 0x10-bytes from the hash is used with the below snprintf for ID0 in sdmc/Nintendo 3DS/<ID0>/<ID1> and nand/data/<ID0>. ID0 is generated by: snprintf(outdirname, maxlen, "%08x%08x%08x%08x", hashword, hashword, hashword, hashword).