One-To-One Relationship
The One-To-One Relationship can be used to store a relation between 2 Business Models.
For example if you have a model "Person" and a model "Car" and a "Person" can only have one "Car" you can use this relation to easily find the "Car" a "Person" is using and vice versa.
Filters
You can specify filters which will be applied when creating/update objects and when filtering.
For example if the model "Person" has property of type Status(Active,Not Active), you can set a filter in model "Car" - Person.Status In Active.
If you do this users will not able to create/update of filter objects of type "Car" where the Person.Status is not Active.
See Model Designer