An identifier is an email address or phone number.
Reklaim wants to give you the highest likelihood of finding any breached or broker collected data so you can take the necessary steps to protect it. To ensure this, when scanning, follow these tips:
Use the "Account" section to add more identifiers, think of all the identifiers you've used to create accounts on various online products/services.
Use identifiers that you have used online for the longest period of time, the more exposure an identifier has had on the web, the higher chance it will be used by brokers or found in breaches.