If you use PI markers for CSH destinations, you need both a map file and an alias file:
When you use PI markers, developers should use the HH_HELP_CONTEXT API, and specify topics by numeric ID. You need the map and alias files to associate their numeric IDs with your symbolic IDs. Each time you convert your DITA document to HTML Help, DITA2Go uses the symbolic IDs to generate an alias file; see §18.11.4 Creating an alias file for CSH links. Usually the developers provide the map file.
When developers use the HH_HELP_CONTEXT API, the application must send a numeric ID, and you must have the map file in your project to interpret the numeric ID. The developers need only the map file. You need only to add to your HTML Help project file the name of the map file and the name of the alias file, before compiling. You can do this via configuration setting; see §18.11.3 Specifying and generating a map file for CSH links.