This version and onward uses an updated method for retrieving the Windows 10 ADK installation files from a XML feed hosted here on scconfigmgr.com and now also modified to support the changes that came with Windows 10 ADK version 1809 where the WinPE optional feature is not a single add-on requiring a separate installer. As for the offline method,
Download ConfigMgr Prerequisites Tool 3.0.4
You can download the tool from the TechNet Gallery, following the URL below.
From this version and onward, ConfigMgr Prerequisites Tool source code will be available on GitHub:
Included in the download from the TechNet Gallery is a PDF with more details about each section of the tool.