Font Sense is a technology developed by Extensis to aid in the auto-activation of fonts in their Extensis Suitcase font manager for Macintosh. When a font is added to Suitcase Fusion, a unique Font Sense identification number is created based on several values, including the font's name, version, foundry and metrics. This number will be the same across different systems. This FontSense ID can be embedded into certain document types. This assures that the correct font to be activated when the document is opened, because Suitcase can distinguish the correct fonts from other fonts with the same name.
References
Font Sense Wikipedia(Text) CC BY-SA