This document is an addition to the card database conventions.
Some expansions contain a subset of reprinted cards from several other expansions. Sometimes these cards feature their original expansion symbol, but have a card number unique to the expansion itself. In all cases these cards will be considered a sub-expansion because:
A list of all expansions (and the name of their sub-expansion) can be found below.
For some releases there’s ambiguity whether the cards should be listed as card variants or as separate expansions. A list of all ambiguous releases and the reason for listing them as such can be found below.
N/A
Cards from the Celebrations Classic Collection feature both their original expansion symbol and card number. This means that these cards should be considered variants, but for the same reasons described in the Sub-expansion vs original expansion section, these cards are considered a subset of the Celebrations expansion instead.
An additional reason for not having these cards as variants, is that it would not make sense with regards to master set completion.