Container format

In these cases, programs usually emit an error message that complains of a missing codec, which users may be able to acquire.

In principle, coding can be changed while the storage layer is retained; for example, Multiple-image Network Graphics (MNG) uses the PNG container format but provides animation, while JPEG Network Graphics (JNG) puts JPEG encoded data in a PNG container; in both cases however, the different formats have different magic numbers – the format specifies the coding, though a MNG can contain both PNG-encoded images and JPEG-encoded images.

In most cases, the file header, most of the metadata and the synchro chunks are specified by the container format.

Container format parts have various names: "chunks" as in RIFF and PNG, "atoms" in QuickTime/MP4, "packets" in MPEG-TS (from the communications term), and "segments" in JPEG.

Modular chunks make it easy to recover other chunks in case of file corruption or dropped frames or bit slip, while offsets result in framing errors in cases of bit slip.