What is a domain holder?
Every domain name is registered in the name of a legal entity (a person or organization). This legal person is the domain holder of the domain name and therefore officially the owner of the domain name. Basically, the domain holder is the owner of the domain name.
In your dashboard, you can create multiple domain holders and assign them to domain names within your account. You can do this at "Settings" -> "Domain holders". Each domain name can only have one domain holder.
Default domain holder
The first domain holder you create will be your default domain holder. You can always assign a different domain holder as your default. The default domain holder will be used for new domain names and new orders, unless you choose a different domain holder during your purchase.