The “a” and “b” are indicators that are used to decide whether the record currently being processed in the “data” step comes from the first dataset or the second one. They are numerical values and will have the value 0 or 1—0 if the current record is not in the dataset to which the indicator belongs, or 1 if it is. So, a record from the first dataset has a = 1 and b = 0, while a record from the second one has a = 0 and b = 1.
Normally, the two (or more) datasets would be different ones, though with the same columns (but again, not always). That is, the usual syntax would be something like “set s1 (in = a) s2 (in = b) s3 (in = c) …;” and therefore I suspect that “set data.deve (in = a) data.deve (in = b);” may be a typo.
If it is not a typo, then the output dataset (“data deve;” in the “work” library—the screenshot is too fuzzy to read clearly) will contain double the number of records in “data.deve” except that the duplicate records will have different values for the “y2” and “wt” variables.
The subsequent “proc logistic” step then fits a logistic model to the data.