Question

What do you understand by relationship type? How is it different from relationship instance and relationship set?

Answer 1

Whenever entities from different entity types are related to one another in a particular manner, they constitute a relationship type. The relationship prepared by between the two entity types vouchers and employees associates each voucher with the employee who prepared it.

Similarly, the relationship authorized by between the two entity types vouchers and employees associates each voucher with the employee who authorizes it.
The degree of a relationship type is the number of participating entity types. A relationship type of degree two is called binary and that of degree three is caller ternary.

A VOUCHER (entity), Authorized by (relationship) a) and EMPLOYEES (entity) together signify a binary relationship. A SUPPLIER (entity) SUPPLY (relationship) PARTS (entity) to PROJECT (entity) signify a ternary relationship because three entities, namely supplier parts and projects are participating in a supply relationship in any transaction.
Relationship present at particular instance of time is known as relationship instance
Relationship sets
$1:1$ (one to one relationship set)
$1:M$ (one to any relationship)
$M:M$ (many to many relationship set)