Difference between associative entity and associative relationship attribute?

An associative entity is the table that associates two other tables in a many to many relationship.

An associative relationship attribute is an attribute of the associative entity that exists because of the many to many relationship.

Here's an example. Let's suppose we have the following tables.

User
----
User ID
User Login Name
User Name
User Password

Permission
----------
Permission ID
Permission Name
Permission Description

Ok, we have a many to many relationship between User and Permission. A user can have more than one permission, and a permission can be shared between many users.

So, we create an associative entity.

UserPermission
--------------
User ID
Permission ID
Permission Granted Time Stamp

The permission granted time stamp is an associative relationship attribute. It would not fit in the User table nor the Permission table. It's an attribute of the association.

In ER diagramming, M:N relationships can have associative entities OR single associative attributes that describe the relationship. The difference is, associative entities must have a unique identifier and associative attributes don't. According to the book, an associative entity must have:

1-all the relationships for the participating entity types are M:N relationships 

2-the resulting associative entity type has independent meaning to the end user and has an identifier 

3- the entity has one or more attributes 

4-the entity participates in 1 or more relationships independent of the entities related in the associated relationship.