Below is a network graph for a shortest path problem.

How the decision variables xij should be defined?

Choices are :

xij is the "cost" of traveling from node i to node j per unit

xij is the "number" of item of interest $($e$.$g$.$ cars$)$ that go from node i to node j

xij is $1$ if the route from node i to node j is taken, and is $0$ if the route from node i to node j is not taken Below is a network graph for a shortest path problem.

How the decision variables xij should be defined?