> It is meant to tell the engine that you want a result set consisting of each DISTINCT row containing > the named columns.

And the result set consists of the single column in the expression
with the DISTINCT applied.

> Or maybe he meant it was stupid to return a result instead of raising an error...with which I agree.

Why should it raise and error?

select distinct substr(zip,1,5), namelast from newslet

Works just fine.