I think there’s some confusion in terms.
@notverydeep appears to be referring to
TTN (TimeTable Notice) 100 which comes into effect in a couple of weeks time as described.
@73Stock appears to be asking about a specific train number (100) and its movements today. This would be referred to generally as T100, although all lines to it slightly differently. I suspect this confusion comes from some of the language used on Intertube, which has been written by a layperson (
@eta) and some of which has been fed back before.
T100 isn’t a scheduled or planned working so it really could be anything - although my educated guess is likely depot turning requirements given it appears to have started and ended from the Boston Manor end of Northfields depot. Indeed it may not even have been the same set of stock both trips. Service Controllers can allocate additional trains whatever number they please, and run the trains as suits them.