In transport stream format, the PES packets are sliced into small pieces, each of which is carried in a transport-stream packet. This is intended for use in transmission over less-than-fully-reliable channels (e.g. broadcast over the air), where a momentary burst of interference or loss of signal will only lose some transport-stream packets. 

  

 In program stream format, PES packets are grouped into ''packs'', each beginning with a ''pack header''. Though calling this a “header” is a bit of a misnomer, since it is effectively a packet in its own right, and in particular it has no length field that includes any following PES packets. The purpose of the [pack header|http://www.mpucoder.com/DVD/packhdr.html] is to 1) contain a more precise clock reference (units of 27MHz instead of 90kHz) and 2) give the decoder some indication of the rate at which it will have to transfer data in order to play the movie.