Your question involved several fundamentals of data step. For details, the best instruction I found is from Ian Whitlock.
In short, 'if _n_=1' demands to read only 1 obs from 'summarydata' without triggering off the 'end of the file' event, so data step can keep moving on reading obs from 'shoes' until it is done. You may have already tested, without 'if _n_=1', data step will stop after outputting 1 obs.
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