What mean XX in MNE02xx/A ? And Is Part Number Unique?

In the article you asked this question from:

In Apple part numbers, the "xx" before the slash are variables that differ by country or region.

(162142)

The part number is the number for that particular model, it is not unique.

iMac (Retina 4K, 21.5-inch, 2017)

Model Identifier:
iMac18,2

Part Number:
MNDY2xx/A
MNE02xx/A