Skimming creates a RuleSet version containing the highest version of existing rules. For each rule instance in a specified RuleSet, the wizard identifies the highest-numbered version and creates a copy with an even higher number that you specify.
To answer your question, the skim wizard takes care of all the available rules in the available ruleset(Including custom created rulesets) of the application.
As per my understanding, if you skim ruleset from 01-01-01 (start) through 01-01-50 (end) to 01-02-01 version. After the skim process completion, if the end user wants to know which start and end version of ruleset was being used to create this 01-02-01 version? -> Then there is no OOTB way to get this information and the end user should be aware of this information while doing the skim process, he/she needs to store this information for future references.
Note: Even the history of any rule that has been skimmed to end version also specifies only "Record skimmed to Ruleset:01-02-01" and doesn't contain from which version it has been skimmed.