A Google advanced search of the SAS support
http://support.sas.com/ website yielded several matches, some SAS-hosted documentation and some technical papers on the topic. Here's the search argument - the site: parameter limits the search:
proc format cntlin other site:sas.com
Another discovery technique would be to create a simple format with PROC FORMAT and VALUE, coding an OTHER= , and then run PROC FORMAT with CNTLOUT= and use PROC PRINT to see the SAS-generated results.
You will want to explore using the HLO='O' variable value, leaving START as blank, and assigning some desired LABEL variable value -- this would be an extra observation.
Scott Barry
SBBWorks, Inc.