QuickBooks Online helps you keep all of your transactions organized and accessible. A register (or account history) lists all of the transactions for an account. I'd be happy to explain how you can view the register in your Chart of Accounts.
The register is called Account History in QuickBooks Online. To find that, click the Accounting tab on the left navigation menu, select Chart of Accounts, then click Account History next to the account you'd like to view the register for.
Here's a couple articles you can explore in regards to the Chart of Accounts in QuickBooks Online: