Virtual cards are special plus cards that are configured to generate special content based on their names. They're not actually stored in the database but are automatically generated wherever you link to or include them.
Examples
There is a card for TikiWiki (another wiki software project), which is tagged with "free, open source":
"Peers with tag" has a right content form that's a search for Peer tagged with "_self":
peers with tag+*right+*content:
As a result, "free, open source+peers with tag" renders as a list of peer projects tagged with "free, open source" ("_self" in this context), even though free, open source+peers with tag doesn't really exist (if you open it up you'll see it's labeled "Virtual" in the upper right corner, instead of offering Edit and the other tabs):
...Free, open source has an inclusion {{+peers with tag}} on it, so the results of this virtual card can be seen there:
Wagn is licensed under the GNU General Public License (GPL). If our GPL license will prevent you from using Wagn, please ...If there were a form that applied to all cards used to tag peers, {{+peers with tag}} could be put on that form so that all such cards would automatically show the peers that they tag.
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