The MFFS Card is blank when crafted, however, placing one in the "Frequency Card" slot of a Forcefield Core will write a frequency specific to that Core to the card, turning it into an MFFS Frequency Card.
The card can now be placed inside of any MFFS Projector(s). As of Tekkit 3.1.2, there does not seem to be a limit to the distance between the Core and its Projectors. Tested at 257 meters and still connected. This will link the Projector to the Core. Note, you must leave the card in the projector in order for it to stay connected. Yes, this means you must craft and write an MFFS Card for every single projector you wish to create. You do not need cards for Forcefield Upgrades.
When a MFDevice is used in wrench mode on a Projector, or if the projector is destroyed with a pickaxe/drill, it drops the card as a blank card, thus resetting the card. The card is lost if it is left in a Forcefield Core when the MFDevice is used to recover the Forcefield Core.