When setting up the D500x, the settings that might cause some confusion are the gain, trigger level, interval and trigger sensitivity. The gain is how much the signal is amplified, so set it low if the bats are near and loud, or high if they're far and quiet. The trigger level is level at which the signal will trigger the device. The interval is the time between recordings in seconds - this is to avoid recording the same bat over and over again (unless you want to of course). The one that's got me scratching my head is the tigger sensitivity, which seems to be how long the signal has to remain over the trigger level to start the device. The options are very high, high, medium, low and very low. I guess the idea is to have a setting that will not tigger the device over short signals, rain perhaps.
When setting it up, the device gives a handy indication of when it will be triggered, so you can jangle keys or loose change and make sure it will trigger but that it's also not going all the time.
For the purposes of testing it in my garden, I've got gain and tigger level set to 30, interval at 0 and tigger sensitivity at low.