Explanation of QRG screen

[myhillman]

Whilst the QRG screen is relatively "self evident", there are some bits that aren't.

1. I've managed to deduce that if the Launchsite starts with "@", it means a volatile entry that can be overwritten by and update from Sondehub. Similarly, if it does not start with "@", it is a fixed entry, and won't be overwritten. Can this be confirmed as correct, and where is it documented?
2. My downloaded Launchsites look like "@U2730244/74". The "@" I understand, the "U2730244" I recognise as the sonde serial, but what does the "/74" mean (and where is this documented)?