When you modify a selection list, you should immediately do a full backup to establish a new baseline.
Assuming that you are using the archive bit method, the incremental backup will only backup those files in the new addition to the selection list with the archive bit on. Those files in the new addition without the archive bit on WILL NOT be backed up.
An incremental backup will only back up files with the archive bit on and nothing else.
You can easily prove what I said is true
1) create 2 directories with some files in them.
2) use 2 jobs to do a full backup of these two directories. This will reset all the archive bits.
3) modify some of the files in directory A.
4) create an incremental job to backup directory A.
5) modify some files in both directory A and B.
6) add directory B to the selection of the incremental job created in Step 4.
7) run the incremental job. You will see that only the modified files in directory B are backed up and NOT all the files in directory B.
The above also holds true if you are using the modified time method.
