If a 100 MB file is reduced to 10 MB, that would be 10X, as in reduced by a factor of 10. It can also be put as a 90% reduction.
Low is configured for speed over reduction rates. For example: Low-Low bypasses the disk based optimization for optimal speed.The best combination of speed vs peak reduction to a remote site is to use High->Low. For peak bidirectional optimization use High<->High, which would be used if the link is likely to be congested in both directions.