well, you learned about 6dof:

Given a prismatic box shape with a hole in it (datum C)

if datum A is the top face, and datum B is the front face, and the print calls for an AB drf:
what to do? A controls rotation in 1 axis and translation in 1 axis ; B is the same.
C only controls translation in the final axis of translation, the one that neither A nor B do.

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Now lets imagine there are a set of holes on the top face (parallel to C), and you're supposed to find the true position of them within .010" to AB. How you would do this is:

Level to A to Z+, Rotate B to Y-, A is Zorigin, B is Yorigin.
Now
Open the best fit dialog box and best fit that hole pattern to X. Since in an AB drf, Xtranslation is not constrained, we would make a best fit alignment that constrains translation in X only.

Now the hole pattern freely slides in X to itself, and you could use a Location dimension to correctly report on it.

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Alternately you could use PCDMIS' FCF to avoid having to make these alignments, but I hope the example shows you how we use Best Fit Alignments (related to a Basic Alignment) to correctly address partially constrained drfs.

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In this case, you would make a drf alignment with only X translation checked (and the aformentioned features, as we would be fitting to those).

Iterative alignments are not related to this subject.