Merchant Account ID
A single Braintree gateway can have multiple merchant accounts to process transactions for different businesses or currencies. Each merchant account within a gateway can be identified by its unique merchant account ID. This is different from the Merchant ID, which is a unique identifier for your entire gateway account.
You can use this ID to specify a merchant account when creating a transaction. If you have a single merchant account, it is not necessary to specify a merchant account ID, but for customers using Braintree to support transactions in multiple currencies, multiple Merchant Account IDs are required.
if you populate the Username field on the Chargent Gateway record, we will send that to the Merchant Account Id in Braintree, to identify the sub account that should receive the transaction. If the Username field is not populated with a Merchant Account ID, then we just send Braintree the default Merchant Account ID.