Thanks Milan for answering.

6. pg 656 - The Lookup Table: "A lookup or control table is
typically a static table with a small row count, which may
be used in a similar way in several contexts. It consists of
a small primary key and a description field, calculation
factor, or some rule that processes need to refer to. ...
Because one row in a lookup table supplies information to
many rows, a slavish adherence to relational analysis rules
often results in FK being bestowed on the lookup key columns
of the user tables."

Q6: Is there some rule of thumb/calculation rule to make
up whether a table is to be considered as a lookup table or
not. A threshold value of the index selectivity ?

Anybody ?