Proxy Cards

On vekn.net there was a discussion (as part of a thread regarding the fan-made set for VtES in 2012), what a proxy card in a collectible card game actually is. Wikipedia has the following definition for proxy cards:

A proxy card is an easily acquired or home-made substitute for a collectible card. A proxy is used when a collectible card game player does not own a card, and it would be impractical for such purposes to acquire the card. This usually occurs when a player desires a card that is cost-prohibitive, or is "playtesting" with many possible cards. When doing intensive training for a competitive tournament, it often makes more sense to use proxy cards while figuring out which cards will be brought to the tournament. Another card is substituted and serves the same function during gameplay as the actual card would.
A proxy can also be used in cases where a player owns a very valuable card, but does not wish to damage it by using it in actual play.

Common use of proxies
Proxy cards can be used in various situations. The rules and restrictions are object of common agreement, or a given policy, and may differ from the above mentioned "fair play requirements".

• In casual games, the players may agree on a common policy of how to deal with proxy cards. This allows to play a higher variation of card combinations and strategies, while keeping a limit on the expenses.
• In tournaments, the organizer may permit a limited number of proxy cards, and define rules about how these cards must look. This policy has become especially popular in games or formats where some vital cards are far too expensive, such as the vintage format in Magic: The Gathering.
• For playtesting. Proxy cards allow a player to test new cards, before he or she decides to actually buy or trade for them.
• In card prototyping. Card developers in companies like Wizards of the Coast use proxies to playtest their ideas for new cards before they are printed.
• Some players create cards based on their own ideas for card themes and mechanics. In this case, however, the term "proxy" may no longer be applicable, as these cannot be considered substitutes for existing objects.
• An extreme form of copying original cards is forgery of expensive cards, which is again outside the "proxy" category.