When updating any collection of sources (eg /usr/ports), cvsup(1) makes use of the related checkouts file in order to perform the updating process in the most efficient and correct way. In this example (/usr/ports), the related checkouts file is /usr/sup/ports-all/checkouts.cvs:. if your base is /usr.
A checkouts file contains information on the current status of your sources—in a way, a sort of “photograph”. This significant information enables cvsup to retrieve updates most effectively. Further, and maybe more important, it enables cvsup to correctly manage your sources by locally deleting any files no longer present in the repository, thus leaving no stale files on your system. In fact, without a checkouts file, cvsup would not know which files your collection was composed of (cf cvsup(1) and the fallback method for details); as a result, it could not delete on your system those files no longer present in the repository. They would remain on your system (stale files), and might cause you subtle build failures or other trouble. For example, this problem is likely to occur if you first update your ports collection several weeks after you got your installation CD-ROMs.
It is therefore recommended that you adopt the two-step procedure outlined in the CVSup FAQ (cf Q12, Q13); in subsequent sections, you will be given interesting and instructive concrete examples.