It is an identifier your character leaves on the things her Awakened Will has touched. When she uses a spell, Praxis, Rote, or Attainment, she leaves little wisps of her identity on that magic
A mage utilizing Focused Mage Sight can recognize those signatures she’s seen before. If the signature comes from a particularly great Gnosis (6+), it offers a bonus to Revelation rolls regarding that Pattern.
For every dot of Gnosis above five, add one die to relevant dice pools. By default, this lasts for a week. However, the bonus dice afforded by high Gnosis fade one per week instead. Once the last die is gone, it fades one week later.
A Signature Nimbus looks the part of a residue from or remainder of the Immediate Nimbus. If your character has a fiery Immediate Nimbus, her Signature Nimbus might be charring and ash, for example. Or if her Immediate Nimbus causes intoxication, her Signature Nimbus might feel like a hangover.
If a mage wishes, she can imprint her Signature Nimbus on an object, place, or person at the cost of one Mana. In this case, add her Gnosis to any rolls to scrutinize that Nimbus. This fades at one bonus die per week. Spending a Willpower point turns those weeks into months.