Send to Device Conversion(?)

[rumplestiltskin]

I suspect I know what happens during the "Send to Device" function but wanted to clarify this with those who have had more experience. As well, I'd like to know about options.

I configured my Kindle Fire HD8 to permit data transfer via USB, connected it to my (silicon) Mac, launched Calibre, then use the contextual "Send to Device > Send to Main Memory" command. I was presented with a dialog asking to confirm a format conversion (to what it does not say). If I dismissed this without approving, no transfer took place. If, however, I approved it, the ePub I wanted to send appeared in my Kindle app on the device within seconds (maybe 15 at most). Obviously, this came from my Mac via USB (the Kindle's WiFi was off), and the book may be read and navigated as expected.

I did examine the files with the "Files" app (I did install the Android Play Store apps so, other than the Kindle app, I use the Fire as an Android device) and found the transferred books as ".azw3" format. Strangely, I can open the book with the PocketBook (Android) app (which sees it as a MOBI) and read it but tapping on any of the TOC entries result in a dialog pointing to the Kindle app (which transfers me over to that app). So I guess Calibre sees my "device" as a Kindle rather than generic Android.

Now the "options" question: Can I configure Calibre to transfer my ePubs to the Fire using the USB "Send to Device > Send to Main Memory" and leave them in the original format? Or is there another command I should be using that I have missed? I might want to use an Android reader app that is happy with ePub (as my entire library is ePub).

Thank you in advance for suggestions.

Barry