I have a Singer XL1000 about 8 yrs old. It has the low bobbin indicator message and beep, and a drop in bobbin with plastic cover. You can set when you want it to alert you, and both sewing and embroidery modes have their own alert settings so you can set them differently. The setting is for how many stitches do you want remaining on the bobbin when you are notified that you're running low on thread.
It gives you an alert beep and message 3 times before your down to your last 5 (or 6?) stitches worth of thread. The last alert, at 5 stitches, gives you just enough thread to tie off and that's it. If you set it to alert you when you have thread for 5 stitches left, then you only get the one alert. Otherwise, it spaces them out. (Ex: You can choose 20 stitches remaining, then you get 3 alerts - 1st at 20, 2nd at 10, and the last one at 5 stitches. Hope this makes sense.)
I really like that I can set it for how far out I want to be signaled. It's a very nice feature. Warning though...the beep can startle you sometimes!