Reference files cannot be sent to the 'External Project Creation' and 'Prepare Packages' Jobs. A separate bundle for reference files must be created and it should not be assigned to the 'External Project Creation' and 'Prepare Packages' Jobs.   This way the reference files will not be included in the Trados Studio package (SDLPPX), but the translator will still receive them as additional files. See the basic settings of bundles for a translation-only workflow below:

And the settings for a translation & proofreading workflow:

Note that it is possible to have more bundles, e.g. separate bundles for TMs and TBs, more bundles for work files each of which can be assigned to Jobs differently etc. The above examples present the simplest configuration possible which includes handling of reference files.