Complex sentence: Because ILBM is based on the Interchange File Format the file consists of chunks, each chunk consists of a 4 byte type, a 4 byte length field and a structure depended on the type of the chunk.
Split into simpler sentences: Because ILBM is based on the Interchange File Format, the file consists of chunks. Each chunk consists of a 4-byte type, a 4-byte length field and a structure dependent on the type of the chunk.