Mysql – How to make Event (like a Party) Sources mutually exclusive

If your tables are already in 5NF then you don't need to think much more about further normalization.

Note however that the principle of full normalization (POFN) is no panacea. There may remain non-obvious update anomalies.

Database constraints will need to be applied. In my experience, designers who start with an ER diagram are great at implementing primary keys and foreign keys but other constraints can be easily overlooked. Other design approaches may result in better constraints e.g. I have a little experience of object role modelling which seemed to work well.

Further normalization should only be considered in conjunction with constraints because 6NF often makes constraints more difficult to implement using today's industrial-strength technologies e.g. none support Standard SQL's CREATE ASSERTION nor CHECK constraints that may include a subquery.

Also consider the principle of orthogonal design (POOD).

The key to understand this case is at the ninth page from Doc-A:

"product" and "customer" are entity types, and "shipped to" is a relationship... the verb "ship" is then converted to a gerund... in other words, the relationship... has been converted into the entity type "shipping"

Hence, the final third form will have "shipped to" converted to an entity by itself, "shipping".

This is necessary because "Shipped to" is a MANY-TO-MANY relationship (although to myself Chen's text is not clear on this).

But, your example does not relate - the relationship "Identified by" is an ONE-TO-MANY (a card belongs to ONE customer / a customer have ONE OR MORE cards issued).

Hence, the model would have the relationship between "Bank" and "Bank card" identified by TWO verbs.

Or, making the model stronger, to register several authentication events, one may turn "Authenticated by" into a entity by itself, "Authenticating" (to register event date, data shared, etc.).