MxCh

MxCh is the identifier for a Lego Island data chunk. They are seen extensively in SI Files to allow interleaving of several different data types.

MxCh chunks contain partial data of various different data types intended to be joined together continuously to form a complete file (an MxDa section). The chunks can also contain solely header data. A chunk will almost never contain a complete file on its own.

Specification
The MxCh header is 22 bytes long and specifies the length of the chunk with other data.

"Split" Chunk?
In addition to the datas being split into chunks, it also appears the chunks themselves can be split arbitrarily and have another chunk of data inserted between them. If a chunk is split, both chunks will have the Flags? section set to 16. They'll also both have the same Milliseconds? value. The "Chunk Size" will be accurate to each chunk's size, but the "Chunk Data Size" of the first chunk appears to be the total size of both chunks' data. The second chunk's "Chunk Data Size" is accurate to its own chunk data size. This is newly discovered information and may not be completely accurate.