How is cardinality described for an arrowed line?

Question

There is an E.R. diagram like below.

My questions are:

1. What is the cardinality for the arrowed line between “teacher” and “offer”? (Or is it possible to talk about cardinality for this diagram?)
2. What does this arrow mean about relation of “teacher” and “course”? (I found out that it is for showing the relation’s way from left to right and means “teachers offer course”. Is it true?)
3. Why is this arrowed line between “teacher” and “offer”, but not between “offer” and “course”? Is there any difference for these situations? If yes, what are the differences?
4. What is the notation type of this arrowed line according to third link below (or according to any other source) ? If it is “Shaler/Mellor”, why is the arrowed line in bold style?

I examined the related questions and link below. But I really confused.

1. entity relationship diagram
2. One-to many relationships in ER diagram
3. Class Diagrams
4. Entity Relationship Diagram

Answer 1

As far as I know, your diagram uses a mixed notation. It's mostly Chen's notation, but he used 1 to indicate unique constraints on a component of a relationship, and variables (M, N, P...) to indicate multiple possible occurrences. Different variables were used per relationship so that numerical correspondence between different roles weren't accidentally implied.

Some online sources (e.g. this one) show that an arrow represents a unique constraint, and a bold line indicates total participation. In your diagram, that would mean mean a teacher must occur exactly once - each teacher offers a single course.

In some examples the arrow is reversed without a change in meaning. It's also possible that the author of your diagram only meant to indicate a preferred reading direction. Without a reference or explanation, we can't be sure.

If the arrow was used to indicate a unique constraint, then it matters on which role its used. An arrow between offer and course would mean each course can only be offered by a single teacher. An arrow on both roles would indicate a one-to-one relationship.

Whether this notation was adopted from data structure diagrams, the Shlaer-Mellor method, or just for a more visual indication of cardinality, I don't know.