There will often be files which should not be added to source control. For example you may not want to add your build files or user option files to PureCM. You can specify which files to ignore using '.pcmignore' files.
If a file has already been added to PureCM then it will not be effected by pcmignore files.
When you first import your files into PureCM a default pcmignore will be created at the root. You can add new patterns to this default pcmignore file or you can create new pcmignore files in a subfolder. These patterns will only apply to this subfolder and folders below.
You can find a good description of pcmignore patterns here.