History Length determines the number of days in the past that are used to calculate the categories of your products.
​The minimum number of days is 30, and the maximum is 365.
You can change this value to better suit your needs.
So that the categories can be calculated taking into consideration last year's sales. For instance all of last year or just six months.
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Here is how you can find and change this:

Go to Settings and under the A.B.C. Classification select Advanced Settings and History length, don't forget to save in the end.
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