B2C SaaS companies that operate in spaces with low barriers to entry, low switching costs and high number of substitutes closely track the performance of monthly cohorts with a cohort as defined by the number of customers acquired in a given month. Such consumers can be fickle and may churn out quickly. Therefore, you must monitor the slope of the retention curve over the subsequent month. One way to report on this curve is to use a set of metrics defined by the percentage of users active as time progresses. For example, the metric, D30, refers to the percentage of active users that were acquired in the prior month, i.e. Month 0, on the 30th day of the following month, i.e. Month 1. You will see every variety of this metric: D7, D15, D30, D60 in days; W1, 2, 3 etc in weeks, and M1, 2, 3 etc in months.