Roger -
Short answer, your statement of "So the file as it is in BD, plus any changes from Mon, Tue, Wed, Thur and Fri are used to rebuilt a version of the file as it was on Fri (end of day)?" is pretty accurate.
Long answer below... :)
In addition to what Perry has said, here's some additional information about VSS and the snapshots it creates.
http://technet2.microsoft.com/WindowsServer/en/library/2b0d2457-b7d8-42c3-b6c9-59c145b7765f1033.mspx?mfr=true
CPS uses the "copy-on-write" aspect of VSS. If it helps, you can think of a snapshot as a "differential" backup. For example, if snapshots were scheduled for 1:00PM and 2:00 PM, that specific snapshot would only contain the differences/changes to that volume made between 1:00 PM and 2:00 PM. The unchanged files on the volume are not in the snapshot; however, they are "linked" via filesystem magic into the snapshot.
So, while the "physical" snapshot contains only the changes between 2 specific times, the entire volume is presented logically to Backup Exec when you do some operation like backup the snapshot.
Going back to your original post, when you backed up *all* snaps to tape, you were effectively doing N *full backups*, because of the logical presentation of the volume that VSS provides us.
Regardless of the amount of differences between a volume at two given times, we've seen that the minimum snapshot size is somewhere arond 300MB. So if one part of a 1MB file changed, the snapshot size will be many times larger than the actual change, because of files sytem overhead.
From a best practices persective, use the "Most Recent Snapshot" or the "Oldest Snapshot" to create Backup Exec Jobs that protect CPS snapshots.
Hope this helps...
Thanks,
Aidan