This metric is relevant in the context of ga:cohortNthDay/ga:cohortNthWeek/ga:cohortNthMonth. It indicates the number of users in the cohort who are active in the time window corresponding to the cohort nth day/week/month. For example, for ga:cohortNthWeek = 1, number of users (in the cohort) who are active in week 1. If a request doesn't have any of ga:cohortNthDay/ga:cohortNthWeek/ga:cohortNthMonth, this metric will have the same value as ga:cohortTotalUsers.