Rex PM allows you to enter and manage multiple accounts, but generally, you have a main trust account that you are working with.
Setting this account as your Default is a great tool to help with easy selection throughout Rex PM.
You can manage your default account using the below steps:
Go to Settings from the main left-hand menu
Select Bank Accounts from the pop-out menu
Click the vertical three dots on the bank account you would like to select
Select Make Default from the drop-down menu
Click Confirm