was the process used to minimize the scanning effects on client systems, while still gaining the majority of the information. For example, you could find winword.exe and by reading the file header properties, Altiris could then determine that MS Office Suite was available. Then it could stop scanning the directory and move on.
So, you got the results you wanted (I have x number of computers with Office 2003 installed), however you were missing installed instances of excel.exe, etc.
That is where file mode comes in. You get EVERY .exe, and more complete data, with the tradeoff of longer scan times and potentially more client impact.