The N-day retention rate shows how many users were retained on a specific day after registration, such as the 5th or 30th day. You can determine each day according to strict calendar rates or sliding 24-hour windows. N-day retention rate is valuable if you expect people to use your product every day, such as a mobile game.
The unbounded retention rate measures how many users returned on a given day or the following day. This retention indicator makes sense if you don't expect people to use your product every day. This number is inversely proportional to your churn rate.
The period-based retention rate measures retention over user periods specific to the frequency of use of your product, such as: Day 1, Day 3, Day 7, Day 14, Day 31.
You can use different retention metrics at different times, but if you are going to compare metrics, make sure you only compare those retention metrics that are measured in the same way. Comparing your unbounded customer retention rate with your N-day user retention rate is like comparing apples with oranges, which will not give you meaningful information when measuring user retention.