This is relevant in the context of a request which has the dimensions ga:acquisitionTrafficChannel/ga:acquisitionSource/ga:acquisitionMedium/ga:acquisitionCampaign. It represents the number of users in the cohorts who are acquired through the current channel/source/medium/campaign. For example, for ga:acquisitionTrafficChannel=Direct, it represents the number users in the cohort, who were acquired directly. If none of these mentioned dimensions are present, then its value is equal to ga:cohortTotalUsers.