I should know this, but don’t.
Was just looking at the Stage 4 TdZ short (C) route – Petite Douleur.
It shows as a 15.4 mile route, but looking this route up on ZI, it appears the lead-in is 6.8 miles of this, and the route itself is only 8.6 miles.
What’s the point of classifying this first section as a lead-in, versus just calling this overall route simply a 15.4 mile route?
In case you are sick enough to do multiple laps. 
Freeride of Petite Douleur, you start by banner at top of Petit KOM, far more sensible.
Laps have to start/finish at an arch. Getting to the first arch from the pen is the leadin and then if you are doing multiple laps each lap is defined as doing the entire route once.
I think I understand what you’re saying, but looking at the route, it looks like a full loop anyway – the lead in doesn’t come into the loop from somewhere extraneous, or do lead-ins not show on the ZI maps? In other words, the lead-in would be part of any additional lap?
The route starts (and ends) at the Petit KOM, so the lead-in is from the starting pens to Petit KOM.
Yes, in this case that is correct.
You have to think about where a route can end, it has to be at a segment arch (Makuri Islands being the exception to this rule). In order to be a looped route that allow for multiple laps, it also has to start at that same arch. Then think about how you get from the pens to that arch - that’s the leadin.
