BTW that was just my first guess, not knowing the details of your
business/agency. But a good place to start, even if imperfect.
FK = Foreign Key

To answer this and your linking question.

Let's say that you just entered a new Source, the city of Las Vegas, NV.
You have an autonumber PK field named SourceID. When you entered that
record, Access automatically gave it a source ID number of 1234.

Now you have your first two datasets: December 2009 Plutonium leaks, and
January 2010 Plutonium leaks. You want to enter these and record where they
came from via a link. There are three steps to linking:

One time: Put an integer (not autonumber) field called
"IDNUmberofSourceofThisDataset" (or "SourceID" as I said before is also OK)
into your Dataset table. This defines the intended use of this field as a
"Foreign Key" ....FK is a USE, not a structure setting in Access like PK.

One time: Open the relationships window and draw a line between this field
and the "SourceID" field in your source table.

When adding those two DataSet Record, write "1234" in your
"IDNUmberofSourceofThisDataset" field.

Later this loading of the field and and use of the links gets automated by
forms.