The number of returning visitors in the Traffic > Unique visitors > Acquisition/Loyalty analysis is calculated based on the comparison period.
We compare the unique visitors of the analysis period (using their cookie ID) with the unique visitors of the comparison period. Matching IDs are considered returning visitors.
For exemple, if you analyze the month of April with a March comparison period, we will consider as returning visitors those that came to the site at least once in March and April. If a visitor came in February and then in April, he will be considered as a new visitor in April, because he did not visit the site during the comparison period (March).