The backup file is a complete copy of your wallet file. It should be compatible with PC Electrum and protected with the password that you've set.
I tried exporting a backup of my electrum wallet on Android and import it into electrum on PC but it didn't work. I assume there is a compatibility issue.Here is the error message I got when I tried to import the wallet backup file:
You can import an older wallet file to a newer Electrum client but not to an older Electrum client.
You're using either of the versions 4.0.2 up to 4.07 in your Desktop while using the latest in Android.
(based from the "storage version" in the error)