C drive Image

Your understanding is basically correct, sector-by-sector is simply copying every byte of data on your partition to a backup file. Its slower and much less efficient than the option of doing a regular partition backup, which only stores the sectors that are actually used, and compresses the data it finds in those sectors.

The primary usage of SBS is in situations where Acronis can't recognize a partition format, therefore can't determine what is actually used, and what data it contains. In that case it simply copies 1s and 0s to another location without any regard for what the values actually mean.

Thanks, so the secondary option - backing by unallocated space - would be even slower then the SBS ?

Yes, it would be slower unless you don't have any unallocated space on the drive. Unallocated space is an area on the drive that does not contain a partition. There can be several on a drive, one, or none, just depends on the partitioning.

In most cases, a standard image is fine. It's faster and takes up much less space.

Thanks,
so to be clear, a standard image is one when I do NOT select
sector-by-sector or the backing by unallocated space check boxes ?

That's correct.