Nand/private/movable.sed

Offset	Size	Description
0x0	0x4	Magic "SEED"
0x4	0x4	This u8 must be zero
0x8	0x100	RSA-2048 signature over the 0x10-byte data at 0x108
0x108	0x10	?
0x118	0x8	Last 8-bytes of an AES engine keyY for 3 keyslots

This file is transferred to the destination 3DS during a System Transfer, the source 3DS then uses a new file retrieved from the shop server. This keyY is the console-unique portion of these 3 keyslots. All of the data under sdmc/Nintendo 3DS/<ID0>/<ID1> and nand/data/<ID0> use these console-unique keyslots. It's unknown where the first 8-bytes of the keyY is loaded from.

Movable.sed always exists on retail, 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. Therefore, movable.sed presumably doesn't exist on development units.

The keyY is hashed with SHA256, the first 0x10-bytes from the hash is used for ID0 in sdmc/Nintendo 3DS/<ID0>/<ID1> and nand/data/<ID0>.