In a number of output formats temporary files are generated as source for the final output format (CHM, ePUB, Kindle, eWriter, Visual Studio Help). By default, these files are written to a folder inside the project folder, but you can change this location if you like. Doing this can speed up the publishing process, particularly if your project is accessed on a network drive by the instance of Help+Manual performing the publishing operation. Publishing will be much faster if the temporary files are created on a fast SSD drive on the same computer where Help+Manual is running.
The temporary files folder is a global program setting that will also be used in command line operations once it has been set in the Program Options.
See Optimize Publish Speed for more details on folder locations and publishing speed. The referred topic is for CHM output but the same principles apply for all output formats where temporary files are generated.
You can find the setting in the Compilers section of the main program options, in View > Program Options > Compilers. The setting applies to all output formats that use temporary files before (CHM, ePUB, Kindle, eWriter, Visual Studio help). The temporary files are written in sub-folders with names based on the project name and the output format.