[Info-ingres] Nonidentifying/optional relationship with NULL FK?

[nikosv]

Searching for "non-identifying relationship where the FK can be null" I stumbled upon the "Nonidentifying/optional" part on page 279 of the book "Beginning Relational Data Modeling"

https://books.google.com/books?id=62CFtFea0NsC&pg=PA279&lpg=PA279&dq=nullable+foreign+key+optionality&source=bl&ots=CM0t0Jfb6L&sig=9F_WdkXta3v-3Uo6Ym8r7UsYJ9A&hl=en&sa=X&ved=0ahUKEwi1v_Sm77rTAhUJOsAKHXM_BMgQ6AEISjAG#v=onepage&q=nullable%20foreign%20key%20optionality&f=false

Is that a valid point? Is it something you came across or have used it?