I see that Maretron’s DSM150, DSM200, and DSM250 displays have a SSC200 configuration setting called "Variation Source:"; what is this?

Maretron's DSM150, DSM200, and DSM250 displays have a SSC200 configuration setting called "Variation Source:", which is used to tell the compass from where it should receive the magnetic variation for subsequent transmission of the heading (HDT) sentence over the NMEA 0183 interface.