This is the Id which identifies a particular setlist track. Used e.g. in Setlist - Fire a specific track. Please see the explanation there:

Albeit there are various functions to fire and edit setlist tracks, the main challenge is to identify the particular track. After a number of pointless tries Gregory confirmed that currently usernumbers for setlist tracks are not implemented. Hence the only way to identify a track is its id. And as it is also not possible to make a track show its id and it's also not shown via Web API the only way to get the id is to retrieve it from the showfile. And finally, as SetList.FireTrack comes from the namespace SetList passed TitanIDs are parsed in the SetList context which fails. The only way to make it work is the string notation SetList.FireTrack(“trackHandleID=1840”).

In order to make it easier to get a setlist track id the showfile parser has been extended to show them (click Show Setlists after uploading your show).

The trackHandleId is an Int32 and is set by Titan.