This variable holds the index of the time line currently associated with the instance. You can set this to a particular time line to use that one, or set it to -1 to stop using a time line for the instance (if no time line is defined for the instance, -1 is returned too). Note that this does not start the time line - for that use the variable timeline_running.
For more information on Timelines see here.
Real (Resource index value)
timeline_index = global.tl;
The above code will assign the instance running the code a time line indexed in the variable "global.tl" if that time line exists.