Additional naming conventions for chunk types are discussed in the next section. The data bytes appropriate to the chunk type, if any. This field can be of zero length. The CRC is always present, even for chunks containing no data. See CRC algorithm .
Chunks can appear in any order, subject to the restrictions placed on each chunk type.
Multiple chunks of the same type can appear, but only if specifically permitted for that type. Chunk layout . Chunk type codes are assigned so that a decoder can determine some properties of a chunk even when it does not recognize the type code. These rules are intended to allow safe, flexible extension of the PNG format, by allowing a decoder to decide what to do when it encounters an unknown chunk. The naming rules are not normally of interest when the decoder does recognize the chunk's type.