Valid only in a callback. Use the SELF handle to get it, as in the following fragment:
myColNum = SELF:LOCATOR-COLUMN-NUMBER.
Gives the column number where the text that caused the current callback ends. The first column in a line is 1. If the current location is an external entity—that is, external to the main XML source—the column number is relative to the beginning of the line in the external entity.