Internal properties are on the person object and custom properties can be specified either with the id of the property or an object that contains the name and property type.
If a property with that name already exists then it will be used with no modification, if it does not exist then then property will be created before the import begins.

WARNING: These are treated as a PUT, not a PATCH. If you include a property in one person record and not another then the one that is missing it will have it removed