_n_ is an automatic variable that holds the count of data step iterations. Here, I use it to execute the first set statement only in iteration one (before anything else is done).
The nobs= and point= options are explained in the documentation of the Set Statement.
Used like this, they do a direct (random access) read of the last observation of the dataset. This prevents a full scan of the dataset, which will improve performance if the dataset is large.
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