A dashed line means that the relationship is strong, whereas a solid line means that the relationship is weak. On the following diagram how do we decide that the relationship between the Room and Class entities is strong. Is it because Room entity has a regular key (non-composite)?
We draw a solid line if and only if we have an ID-dependent relationship; otherwise it would be a dashed line.
Consider a weak but not ID-dependent relationship; We draw a dashed line because it is a weak relationship.
Weak (Non-Identifying) Relationship
Entity is existence-independent of other enties
PK of Child doesn’t contain PK component of Parent Entity
Strong (Identifying) Relationship
Child entity is existence-dependent on parent
PK of Child Entity contains PK component of Parent Entity
Usually occurs utilizing a composite key for primary key, which means one of this composite key components must be the primary key of the parent entity.
In entity relationship modeling, solid lines represent strong relationships and dashed lines represent weak relationships.
The relationship Room to Class is considered weak (non-identifying) because the primary key components CID and DATE of entity Class doesn't contain the primary key RID of entity Room (in this case primary key of Room entity is a single component, but even if it was a composite key, one component of it also fulfills the condition).
However, for instance, in the case of the relationship Class and Class_Ins we see that is a strong (identifying) relationship because the primary key components EmpID and CID and DATE of Class_Ins contains a component of the primary key Class (in this case it contains both components CID and DATE).
来源：https://stackoverflow.com/questions/16675127/how-to-identify-a-strong-vs-weak-relationship-on-erd