Hi @kngu022,
During the compilation phase (before execution), the Program Data Vector is initialized with the variables in the order they are encountered in your code.
So in your example, the first variable name appears in the INPUT statement (IDnum), followed by Item, Instock and Backord.
In the next statement, a new variable Total is mentioned so it is finally added to this PDV.
15-19, etc. are just 'indicators' for SAS to find where the value of the corresponding variable is.
Hope this is clear.
Best,
E.g.:
DATA update1;
INFILE datalines;
INPUT IDnum $15-19 Item $ 1-13 Instock 21-22 BackOrd 24-25;
Total = instock+backord;
datalines;
AAAAAAAAAAAAA BBBBB 11 22
;
RUN;