PEC (Profile Embedded Content) files are used by the Xbox 360 as an additional layer of security on profiles. Certain GPD files are relocated inside the PEC file. The PEC file stores information on avatar clothes/items, and is just another STFS package. With this said you must properly rehash and resign the PEC file to avoid the Xbox detecting corruption of the package. Rehashing is the same as all STFS packages (besides the starting block), but resigning is different (view below).
|0x0||0x228||Console Security Certificate||Signed|
|0x228||0x14||bytes||SHA1 hash from 0x23C-0x1000|
The Console ID at 0x275 must match the Console ID located in the Console Security Certificate, otherwise the Xbox360 will see it as a corrupted file.
The signature located in the Console Security Certificate is signed using the hash at location 0x228. (SHA1 hash from 0x23C - 0x1000)
From 0x1000 the rest of the file is the standard block portion of STFS, with data block 0 starting at 0x3000, and hash table 0 at 0x1000/0x2000. The PEC file always has 2 hash tables (type 1 package).