As can be seen from the reference below, the stem distance is the distance between the depot and the first delivery. This is followed by inter-delivery distances and the return to depot distance. In particular, refer to the diagram above the text quoted.

https://www.logisticsbureau.com/cost-to-serve-part-2/
"A delivery route or ‘run’ can be thought of as taking place in two parts. The stem distance, which is the driving to and from the delivery area, and then the actually stop / start of the delivery operations. So you would allocate a part of the fixed cost and variable cost to each drop in the delivery sequence. Maybe dividing the fixed costs by the number of deliveries for example and then spreading the variable costs across all the deliveries based on the size of each order. This way you get a true cost allocation that takes into account smaller and larger orders."

Stem distance which is the distance to and from a delivery zone (stem mile)

distance aller retour en km d'une zone/D'un lieu de livraison