Both standard interpretations of the Lindley recursion's variables are valid

Let $D_n$ be the waiting time of the nth customer in G$|$M$|$1 queue. Consider the recurrence equation $D_{n+1}=D_n+V_n-U_n$ if $D_n+V_n-U_n\ge0$, $0$ otherwise. Which of the following interpretations of the variables in the above equation are valid? (i) $U_n$ is the interarrival time between nth and (n+1)th customer. (ii) $D_n+V_n$ is the departure time of the nth customer from the system.