Is DPMO different from ppm?

There was an interesting discussion on DPMO in one of the Lean Six Sigma seminars I was conducting.
We were talking about the role of defect opportunities and it impact on DPMO.
DPMO is inversely related to defect opportunities. As defect opportunities increase, DPMO reduces and
vice versa.
Moreover, defect opportunities are user defined i.e. the number of opportunities for defects on a single part
are specified by the user. A single part can have multiple defect opportunities.
The defect opportunities however cannot be greater than the total number of defects actually observed.
This led to the discussion among the attendees as some of them were trying to understand this.

Also, some attendees were comparing DPMO with ppm and were asking which is a better metric for reflecting
the true level of quality.
ppm is a straightforward metric that finds the number of defective parts in a million.
DPMO on the other hand is dependent on the defect opportunities defined by the user. The user can impact the
DPMO values by modifying the defect opportunities.
A comparison between DPMO and ppm is given in the article DPMO versus ppm by SYBEQ.