SQLAlchemy relationships, when to use which relationship?

I have just started learning Flask and using the flask-sqlalchemy package. I have a schema where roles(id, name) and users(id, username, password, role_id) and role_id is a foreign key for roles.id. One to many relationship as one role can be assigned to many users but every user will have a single role.

In the following code,

class Role(db.Model):
    # ..
    users = db.relationship('User', backref='role')

class User(db.Model):
    # ..
    role_id = db.Column(db.Integer, db.ForeignKey('roles.id')

Here, I have established the usage of role_id, but why do we need db.relationship? What is the use of the relationship options and where to use what:

1. backref


2. primaryjoin


3. lazy


4. uselist


5. order_by


6. secondary


7. secondaryjoin