What does main account mean and how do I choose it in BT Pay Web?

The main account is the default account from which payments are made.
If you have multiple open accounts, you can set one of them as favorite:
1. Access the "Accounts" menu.
2. Choose the desired account.
3. In the right part, press on the submenu "Account settings".
4. Choose the option "Preferred account".
5. Select the ⭐ icon to mark the account as favorite.
6. Press the "Confirm" button at the bottom of the page to save the change.