From the docu for the OPEN Function
The OPEN function opens a SAS data set, DATA step, or a SAS SQL view and returns a unique numeric data set identifier, which is used in most other data set access functions. OPEN returns 0 if the data set could not be opened.
From experience:
The first time you use the open() function in a SAS session, the function will return a value of 1 if successful. Any time you execute the function again in the same session the return value gets incremented by 1.
Think of this value as a pointer to your opened data set that you then use in other functions.
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