When displaying timeline information for an Individual, Family
Historian will display facts (events and attributes) from a number of
different records, potentially. Exactly which facts count as
timeline facts is something that can be configured in Preferences
(General tab). This function will return the numeric order of
each such fact, starting from 1. It will return NULL if called
for a fact that is not a timeline fact for the given Individual (given
the current settings in Preferences).