Voting Sigil
A Wizard's Sigil usually refers to his unique style and identifying quirks of magic. If even a trace of his magic remains, it is possible to detect and identify a wizard's magic as his own.
HoHS introduced the Flourish as an artistic refinement of the caster's sigil through the use of Finesse. It does not change the sigil for magical and identification purposes. See HoHS 60.
The Shroud Magic spell can be used to disguise the wizard sigil of a spell (ArM5 159; see also HoHTL 71).
Another, political reference, is to the voting sigil used by the magus (ArM5 86).
References
Legacy Page
The history of this page before August 6, 2010 is archived at Legacy:sigil