Any event that has a "?" at the end of it is called a filter event. What it does is give you a way to programmatically tell LabVIEW to ignore the event. If you click on the properties at the right hand border of the event case, you'll see one called discard. If you wire a False to it, the event will behave just like a non-question mark event. If you wire a True to it, you will have captured the event, but will tell LabVIEW to ignore that the Mouse was clicked down. If you were clicking on a button, it would be a way to make LV act as if the button wasn't actually clicked.
A good use for a filter event is for a Panel Close? event. If someone clicks on the X of a window, LabVIEW would just go and shut down the window aborting the code. Often you don't want to do that, perhaps you want to save data and close off DAQ and serial devices nicely. Think of how if you click on the X in Microsoft Word, it doesn't just shut down and you lose your document. It asks you if you want to save your changes. With Panel Close?, you can capture the event of a button being clicked, but wire a False to the discard telling LabVIEW to ignore that the button was pressed. Then you can move on and execute a sequence of code to initiate a nice orderly shutdown of your VI.
