Our mini-series of hints and tips from past decades continues with 1999...
Appearing in the same issue of The Missing Semicolon (TMS) was i) an advertisement for Year 2000 Support (do you remember "Y2K"?), and ii) an article entitled "Decoding the _TYPE_ Variable (in PROC SUMMARY and PROC MEANS)" on page 5.
The issue in question was June, 1999: Volume 2, Number 2. The Missing Semicolon, published by Systems Seminar Consultants, has been informing SAS practitioners since June 1998 (just a few months after the first edition of VIEWS News was put to print and posted to its subscribers). It continues to this day and offers free subscriptions to all SAS practitioners.
Contemporary users of PROC MEANS and SUMMARY have the benefit of the CHARTYPE option, but the principles of understanding the values of the numeric _TYPE_ variable or the 1's and 0's of the character _TYPE_ are the same.