If a file in your project has properties that differ from the ones set in the project's properties, the file properties take precedence over the project's properties but not over any directives specified in Additional directives that have an IDE equivalent. This might result in receiving unexpected build issues at build time.
To help work around such issues, Visual COBOL you to convert any directives specified in Additional directives to their project's properties equivalents. With the Check Additional Directives for project properties option checked, when you open a solution Visual COBOL reviews all directives specified as Additional directives for the projects in the solution. If there are directives that have IDE equivalents, you receive an Update Project Properties dialog box that recommends converting those directives to project's properties.