Please check http://www.isapirewrite.com/docs/#conffile
The individual configuration files should be named httpd.ini and could appear in physical root directories of virtual sites. All configuration files have the same format. And it is the format of a standard Windows INI file where settings are broken by sections. The only section allowed in this version of ISAPI_Rewrite is [ISAPI_Rewrite]. All directives should be placed in this section and each directive should be placed on a separate line. Any text outside this section will be ignored.
httpd.ini file example:
# This is a comment
# 300 = 5 minutes
# Block external access to the httpd.ini and httpd.parse.errors files
RewriteRule /httpd(?:\.ini|\.parse\.errors) / [F,I,O]
# Block external access to the Helper ISAPI Extension
RewriteRule .*\.isrwhlp / [F,I,O]
# Some custom rules
RewriteCond Host: (.+)
RewriteRule (.*) /$1$2 [I]
When ISAPI_Rewrite parses configuration file it creates error log file named httpd.parse.error in the same directory where parsed file is located. Only obvious syntax errors could be detected during parsing. Complex syntax errors and logic errors could be found only during rules execution. So, they will not be logged into the httpd.ini. It is recommend to check a new rule with the Regular Expressions Testing Tool before putting a rule into a configuration file.