Mixpanel's unique identifier is called distinct_id.
Mixpanel can keep track of actions in your application right down to the individual customer level. This is done using a property called distinct_id. The property can (and in most cases should) be included with every event you send to Mixpanel to tie it to a user. Distinct_id plays a vital role across most Mixpanel reporting. Here's how:
Segmentation / Trends / Formulas
In the Segmentation, Trends, and Formulas reports, distinct_id is used to determine uniqueness. So when you filter a report down to look at only uniques, Mixpanel is showing you the count of unique distinct_id values that sent that event. When you view averages, it is showing you the average number of events sent per distinct_id per hour/day/week/month.
In the Funnels report, distinct_id is used to follow a given customer through a funnel. When the report is calculated, we take the count of unique distinct_id values that sent the first event. We then follow that pool of ids through the subsequent events, calculating the drop off as we go.
In the Retention report, distinct_id is used to follow a cohort of users through time. When you select a value under "show me users who did X," we collect the entire pool of unique distint_id values that send that event on the given day/week/month. We then follow that group of distinct_ids forward in time to see if there are any the events you specified in the following day/week/month long buckets that were sent with that id. If so, we include that id when calculating the percentage that we display for that cell in the chart.
Distinct_id plays a dual role in People analytics. Firstly, it is the unique identifier for that part of the tool. Data sent to the people section of Mixpanel is dispatched to a distinct_id. If the id already has a People record tied to it, then the call modifies or updates the existing entry. If the id does not already exist, a new entry is created for that id. The distinct_id associated with a People analytics entry can be seen by inspecting the URL when in the profile view (click a user in the explore report).