DIH_Y2 is based on Y2 claims. If the claims for Y2 have not been provided, it's because you don't need them. In other words, the member was no longer with HPN at some point in Y3 (so there is no DIH_Y3), and since you need to use Y2 claims to predict DIH_Y3,
but there is no DIH_Y3, the Y2 claims are not provided.
The claims in Y1 are used to predict DIH_Y2, whereas in the above you are discussing LOS in Y1 in reference to DIH_Y2. They are in different years. DIH_Y2=3 means there were three days in hospital at an emergency department or inpatient hospital.
The key point is that t if there is a DIH given, there were claims for that year. But those claims may not have been provided (because the member was no longer a member of HPN at some point in the following year). And note that LOS is not equal to DIH, necessarily.