When setting up your Megaventory connection with your Magento setup you have the chance of defining the Supplier Attribute Code (last field in image below):
At any point either before or after the initial sync with Megaventory, you can create an attribute code in Magento (or use an existing one) and fill in that Supplier Attribute Code in the Megaventory Settings in Magento above.
This essentially means that that product field in your Magento setup can be used to fill in the Supplier of that Product. If the Supplier is filled in in Magento, it will be passed from Magento to Megaventory and become the Main Supplier of that product in Megaventory.
Note, the Main Supplier in Megaventory does not sync to Magento.