That would be my guess (1 to 9) with the Legacy system but making a train with TMCC engines or mixed?
The Legacy manual points out the 1 to 99 for all Legacy engines.
The old TMCC manual states 1 to 9 but do not duplicate any Track numbers you are using for Powermasters, etc.
I find a lack of completeness in the Legacy manual as to dealing with TMCC engines.