The Classic type of User retention report shows the percent (or the number of users) who came back in specific days.
The report supports Day 1, Day 2, Day 3, Day 4, Day 5, Day 6, Day 7, Day 14, Day 30, Day 60 and Day 90 retention. If the player returns to the game in one of these days, he will be counted in the appropriate column. In other cases, his logins will not be counted.
For example, the player joined the game on 01'July. Then he played the game on 03'July and on 09'July.
Classic retention for this case shows the data for Day 2 retention only: this column reflect the playing on 03'July. The playing on 09'July doesn't influence the report.