A header `packet' is a group of headers relating to one aspect of an observation. When the observing system is configured, the headers to be collected are chosen by whole packets: if any header in a packet is present, the others are also present. Consequently, cross references between headers in the same packet are safe, but references to headers in other packets are not, as that packet may not be present for a given observation.
There is one packet for each sub-system, plus one general packet -- OBSVATON -- describing global aspects of the observation. The packets are linked together as appropriate for a given instrument configuration to provide the complete set of header information. The software that generates headers should write them in the order given below.
Division into packets is only significant during the collection of headers. No alterations to the header format are made to delimit packets within the FITS file.