PK's enforced via constraint or unique indexes?

I just want to confirm something about DB2/400...  99 Percent of the tables
I have on our AS400 are JDE OneWorld tables.  Now, JDE does NOT use Primary
Key constraints.  Rather, they use unique indexes to enforce integrity.

Now in the MS SQL world this is a virtual non-difference because Primary
Keys are enforced via unique indexes anyway.  Typically, the only advantage
of a PK contstaint is that is shows up with a specfiic type of object
attribute - so it can be useful in writing code.  And, there might be an
inherent tendancy for an optimizer to favor PK constraint (indexes) for
joins (another simple benefit of PK constraints over unique indexes).

Does DB2/400 act similarly, or is there NO benefit to using a PK constraint
over a unique index?  Also, if DB2/400 does use unique indexes to enfore PK
constraints, I should be able see it as a 64K/page index used within the
"Visual Explain" tool right?

This question came up because I've got a developer that has created a new DB
schema.  In this schema each table has a PK constraint defined AND an
identical unique index defined for the PK columns- which I expect is
redundant.  Yet, in Operations Navigator I only see the unque index definded
as a SQL index object and I cannot view the constaint as an object anywhere.

Thanks in advance.

Ryan