None of these options are important for the normal use of Drive Snapshot; usually you should use the default settings.
Disk image files can become rather large, if saving big partitions. However they should not be larger then certain limits, else they will not be usable by DOS (limit = 2047 MB) or will not fit on a CD-R/W (Limit 650-750MB).
So Snapshot will cut the disk image file after a preset size (default=650MB), and create several files with extensions ".SNA", ".SN1",".Sn2",..
If you know you have larger CR-R's (or even DVD's), you can increase this limit here.
This is relevant for FAT16 partitions with a large cluster size only.
Each file on your disk reserves more space, then it really uses, because disk space is reserved, based on a fixed cluster size. Usually, Drive Snapshot will save complete clusters into the image file, which includes also the unused space after the file. As the cluster size can be rather big for FAT16, saving only the used space can lead to as reduced disk image size.
The problem is, that the determination of the exact size of each file takes a certain time ( typically ~10 seconds), and an application could change some files during the scanning time.
This would lead to the problem, that the disk isn't entirely consistent, but rather contain some files with the file size of 9:00:00, and with others of 9:00:05 saved.
For this reason, please use this option only if you know that the disk is 'quite' at the time of the backup start for ~20 seconds; then however it's completely save to use this option.
A different possibility to solve the same problem is Command line - ClusterZap ; this will simply wipe everything unused behind the real data with 0's, which are very nicely compressible.
This saves each and every cluster on the disk, ignoring the free space information.
This has it's use, if you are trying some 'Data Recovery' Tool to recover some data from a damaged disk, but want to be able to restore the original state for the case that the 'recovery' fails.
Saves these settings to be uses as new default settings.