Base Time Identities are how Bramble ensures that scheduled work time data being uploaded into the Connector is populating the Base Time section of Member Plans in Bramble correctly.
The first time you upload a new file for a given Workflow and Source, you'll need to complete the initial Base Time mapping via the Base Time Identity Search screen under the Data menu option, as this table only populates once that step has been completed.
Once you have completed the initial mapping, and have successfully uploaded a file, you can use the 'Base Time Identity' option under Configuration to manage those mappings ongoing.