AssumeMerchant is an identifier that identifies a merchant when multiple merchant IDs (MERCHANT_ID) are associated under one ClientID. This can be set in a header or in a query to identify which merchant the payment is being made at.
Please also see the following article.
Note: Regardless of the number of merchants you handle (single or multiple), be sure to set an identifier (assetMerchant) to identify the merchant at the time of payment request.