This is what we refer to as the aggregate score. We are able to create this percentage based on the number of words that have been copied from at least one source and divide it by the total number of words in the original text. 


You can always look at each result and see the similar percentage and words individually. 
https://help.copyleaks.com/support/solutions/articles/35000152973-how-is-the-percentage-at-the-top-of-my-similarity-report-calculated-
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