Difference between Primary Key and DDS Unique index?

["Lennon_s_j@xxxxxxxxxxx" <lennon_s_j@xxxxxxxxxxx>]

I was experimenting with creating tables using iSeries Navigator.  (It 
has a pretty good interface.  Right click on Schema/new/table.)

I wanted to add a foreign key constraint to an existing file defined 
with DDS and having a unique key.   iSeries Nav would not show the DDS 
file as being available in the drop down to use as a constraint.

However, if I manually added a primary key constraint to the DDS file 
from the command line, the DDS file then became available to select for 
a foreign key in iSeries Nav.

A DSPDBR showed no additional LF had been added so I assume the primary 
key was using the unique index, but it looks like a unique DDS index is 
somehow different from a primary key, at least from iSeries Nav’s point 
of view.  (V5R4 version of iSeries Nav.)