The COBOL= parameter influences how the pre-compiler expands IDMS DML statements according to the version of Cobol being used.
The IDMS DML Reference for COBOL manual is located at DML Reference for COBOL.
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The COBOL=1/2/85 and other parameters are documented at Passing Parameters to the Precompiler.
VS Cobol has been out of support for many years and if Cobol II or LE or something higher is being used, then you would need COBOL=2, which is the default.