bg fld == background field, cd fld == card field
Every card has two layers, the background layer (which can be shared by several cards) and the card layer. A background is something like a master slide in Keynote. Text fields that you create on the background layer may (if their sharedText is FALSE) have different contents on each card.
E.g. when you make a database, you can define the different record fields for a database entry as background fields, then enter a different person's details on each card to use it as a database entry.