Originally Posted by
KRviator
IIRC, threshold data (or intersection data) is used by some FMS' to do a last-minute update to the IRS position prior to letting it navigate on its own.
More typically that's done at the gate (most airports publish stand coordinates and/or display them where the crew can see them).
I guess that it's done more easily before you start moving, on the assumption that the IRS isn't going to drift too much on the taxy out.