All ONTRAPORT accounts include at least one User, the Administrative User. If you only have one User in your account then the Contact Owner is that Admin User.
But if you have two or more ONTRAPORT Users you can assign one of them as the Contact Owner. A European sales rep you have as a User in ONTRAPORT might be the Contact Owner for all your contacts in the EU.
You can use the Contact Owner as the "From" email address. When you send an email from the Contact Owner, the business information at the bottom is taken from that User's profile in ONTRAPORT. Many of our clients use this feature to run two or more unrelated businesses in a single ONTRAPORT account, even if they are really both of the Users. They create a second User login and build the profile with the specific business information they want to appear for that business.
You can use the Contact Owner in Rules to set Tasks, notify the Contact Owner of an action taken by a contact, restrict activity by that User in regards to contacts with a different Contact Owner, etc.