I'm aware of the iTunes-specific iTunSMPB (for gapless playback) and iTunNORM (for Sound Check) data in the moov.udta.meta.ilst.---- atoms.

iTunes also uses another moov.udta.meta.ilst.---- atom, "Encoding Params", which has the following structure in its data area:

For AAC:
CODE
00000000  76 65 72 73 00 00 00 01  61 63 62 66 00 00 00 02  |vers....acbf....|
00000010  62 72 61 74 00 03 e8 00  63 64 63 76 00 01 07 00  |brat....cdcv....|

For ALAC:
CODE
00000000  76 65 72 73 00 00 00 01  61 63 62 66 00 00 00 03  |vers....acbf....|
00000010  76 62 72 71 00 00 00 00                           |vbrq....|

When these atoms are set, iTunes will correctly display the target bitrate and mode for the file.

I've deduced that for AAC, acbf is the bitrate mode: 1 = CBR, 2 = VBR, 3 =? VBR constrained. brat is the target bitrate, but I've no idea what cdcv is.

Does anyone have any more info on the specifics of these atoms?