Each time StyleCop is run, it will cache the results of the analysis onto the disk. The next time StyleCop is run on the same project or file, the tool will determine whether any changes have occured within the file since the last analysis. If not, the previous results are read from the cache. For larger files and projects, this can improve the overall performance of StyleCop.
In some unique situations, it may be desirable to prevent StyleCop from writing results cache files for a particular project. This can be controlled by navigating to the Results Caching tab on the settings dialog for a particular project, and unselecting the "Cache Source Analysis results" checkbox, as shown below: