The resellers' owner is used in dashboard cards (e.g. the availability key metric), in managing promotions and to easily confirm account ownership.
Setting is simple.
You will need to have your owners as users (speak to your system admin to add users).
Navigate to a reseller and scroll down to the Details section in the left-hand side bar
Click on the owner and a dropdown will list all users in you organisation, select and you are done.
This reseller will now be used in dashboard cards such as the KPI->Availability or RRM->Promotion cards, in reports playground. The owner will also be shown when viewing resellers.