What is the purpose of the .hdr file when using up2date

[BFD]

When using up2date, I notice that there are two files downloaded for 
each update: a .rpm file and a .hdr file.

What is the purpose of the .hdr file?

If I want to save on CD all the updates that I have applied to my system 
since I installed Fedora Core 1 that are now in /var/spool/up2date, do I 
need include the .hdr files or just the .rpm files?

BFD