CKing@nyiso.com writes to the NYISO_TECH_EXCHANGE Discussion List:

Joe, Don,

As Joe indicated in his TIE response, voltage limits and voltage-collapse
limits are studied off-line using detailed AC power-flow techniques.  The
resulting limits (in kV) are translated into a MW transfer limit  for the
flowgates affected by the original voltage limit.  To the extent that these
surrogate limits reflect the expected voltage performance of the system,
then SCUC/BME and SCD will secure the system accordingly.  These rely on DC
powerflow techniques, which by their nature are Blind to voltage problems.
When this occurs, OOM is one tool that can be used to secure the system.

When the realtime operation deviates from the conditions assumed in the
analysis used to determine the voltage limits, the use of OOM and/or other
measures may need to taken by the Operators to secure the system

I hope this clarifys the situation.

Chuck