|0x4||0x4||When u8 is non-zero, this indicates that the additional 0x20-bytes AES-MAC block at the end of movable.sed exists. If u8 is zero, then u8 , , and  must be zero as well.|
|0x8||0x100||RSA-2048 signature over the 0x10-byte data at 0x108|
|0x110||0x10||AES engine keyY for 3 keyslots (High u64 is unsigned)|
|0x120||0x20||This data is written to the file when doing a System Format. The original movable.sed from the factory is only 0x120-bytes. The last 0x10-bytes in this block is an AES-MAC over a SHA256 hash, using the same keyslot used for NAND dbs. This hash is calculated over the first 0x130-bytes of movable.sed. This AES-MAC is verified is during movable.sed verification(before RSA verification).|
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 and development units(written to NAND at the factory), however if reading this file fails(svcBreak would be executed if the file-read code-path return value is 0xC8804464) 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. On development units the code-path handling movable.sed would execute svcBreak if file-reading(regardless of error-code) or verifying the RSA signature fails(this would brick the 3DS), RSA verification failure on a retail unit here would also cause a brick.
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).