Modality is of two types. Required and Optional
If there is relation between entity instance and it is required then Modality will be 1 (Represent by Dark line in ER Diagram)
If there is no relation between entity instance and it is optional then Modality will be 0 (Represented by Dotted line in ER Diagram)
In above ase, it said there is no explicit need for relationship to occur, then consider it as optional, so modality will be 0.