Item and piece in OPC

[MURATA Makoto]

Dear colleagues,

There are four similar (if not identical) terms in OPC.  They are
  1) logical item
  2) physical package item
  3) ZIP item, and
  4) piece

The relationship between any two of them is 1:1.

Then, how are they different from each other?  Here
is my understanding.

- "Logical item" is independent from
  the underlying physical representation

- "ZIP item" is dependent on the ZIP format

- "physical package item" is either "ZIP item"
   or XXX item, where XXX is a particular
   physical representation

- "piece" is used only when interleaving is involved.
   It is not clear whether "piece" is a logical item,
   ZIP item, or physical package item.


Regards,
Makoto
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