@SHANTHI A  According to the documentation Usage statistics for your Pega Platform systems provided by Pega Predictive Diagnostic Cloud   this is the definition:

Unique users

The total number of unique users that accessed the application in the specified time frame.

Depending on the specified time window, PDC counts unique users only once per day or once per hour. For example, if the same user visits the system five times in the same day within an hour interval, and PDC aggregates the results by the day, all visits count as one unique user. However, if PDC aggregates the results by the hour, the same user is counted five times, once for each visit.

You can export the data from these charts into a single .csv file with separate sheets for each chart by clicking Export chart data.

At the bottom of the landing page, you can find hourly snapshots of the data from the charts for each application on each node.