Yes Ben Spruce seems right, I'm guessing its a pressure of how much gas you have in the bottle and when you get low the needle will work its self to red
You will have to let us know as I could do with one, never know when I'm gonna run out until it does
The trouble with those gauges Danny is that they rely on pressure to tell you how much gas is left and are very inaccurate.
The liquid gas turns to vapour in the top of the cylinder and stops when the pressure gets to a certain level. That pressure keeps the remaining gas in liquid form. When you draw some vapoured gas off for your boiler the pressure drops inside the cylinder allowing a little more liquid gas to turn to vapour.
The process works very well until the end when there is hardly any liquid gas left and by then you are just about to run out of gas. This is an issue the LPG tanks used in motor homes still experience (see RV forum complaints), so over time owners get pretty good at estimating how much gas they have left. My dad was pretty good at doing that over 50 years ago with his gas bottle in our caravan when were were kids.
This is why the safefill gas bottle is a good idea imo as you can see how much gas you have left in the bottle. (Unfortunately, Morrisons LPG garage in Redcar 5 miles from where we live won't allow them to be filled there now either.) If you have a fitted LPG tank then topping up when you are passing a garage is easy.