According to our documentation[1] following selection methods are available:
1. SELECTION_BROWSE
2. SELECTION_DROPDOWN

However SELECTION_DROPDOWN is not supported by UI. It is implemented in kernel [2]

[1] https://doc.ez.no/display/EZP/The+RelationList+FieldType#TheRelationListFieldType-Properties
[2] https://github.com/ezsystems/ezpublish-kernel/blob/master/eZ/Publish/Core/FieldType/RelationList/Type.php