The Significator is a SpiritCard(tm) that represents the Seeker in the world of the SpiritCards. The Significator card allows the SpiritCards to 'see' the Seeker as they see in their world. It allows the SpiritCards to see a representation of the Seeker's heart but not the Seeker's actual heart.
The Reader picks the Significator SpiritCard based on the Seeker's picture, profile, introductory statement and brief chat. This card is the heart of the reading.