M4B Merging Behavoir

[c010rb1indusa]

When I merge 2 M4B files together into 1 M4B file, is this software re-encoding the audio to accomplish this or is it a true merge aka 'remux'? Ideally I'd prefer not to re-encode the audio even though quality loss will be minimal for spoken audio.

Thanks for your support and my apologies if this had been clarified elsewhere.