What is unbalance reduction ratio?

Balancing machine instrumentation indicates the level of unbalance on the component being balanced. A good machine can indicate a value and when a correction equivalent to the value shown is executed the next resultant residual unbalance should be below 90-95% of the first indicated value. For example, on the first run, if the machine has shown say 50 gram at a given location. When a 50 gram weight is added at that location and a residual unbalance reading taken, in case the machine indicates a value of 5 gram and below, then it is said that the machine is having an unbalance reduction ratio of about 90%. For more details refer to the ISO standards 1940, 2953 etc.