Can someone please explain?
From the documentation:
Yes, PROC DATASETS (along with PROC SQL and PROC IML and others) is a fully interactive procedure. That means that statements are executed as they are parsed. This is different from procedures that use interactive "run groups" (such as PROC REG and GLM) in which a RUN statement executes a block of statements. See the section "A RUN statement in a fully interactive procedure" in the article "Do you write unnecessary SAS statements?"
Adding to the answers of @Kurt_Bremser and @Rick_SAS, the article PROC DATASETS; The Swiss Army Knife of SAS® Procedures is a great introduction to the procedure. Also it contains a section on the syntax and how it is structured with Run Groups.
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