If you mean show a white for last lap, that's not what the white flag is used for in SCCA Club Racing. The white flag has just one meaning - a slow moving racecar or a moving emergency vehicle is on course ahead of you.

The GCR does not require any signal to indicate the last lap.

Once a race has completed half distance, if its found necessary to stop the race, it is considered complete. A more detailed wording is found in the GCR, but I don't have it in front of me to quote the section.

Bob Hudson
Atlanta Region