Right angle tactile switch question.

I have been working on a project that uses right angle tactile switches. I looked at the datasheet to see which pins were positive/negative, but I don’t understand it. I have included an image and the link to the product is: http://www.mouser.com/ProductDetail/Omron/B3F-3120/?qs=%2Fha2pyFaduh%2F%2boBDD%2bux0k7ZLv8aIzzSl%2FwBM5t3thZtKFJrpiLYoQ%3D%3D.

Pins 1 and 2 are easy enough to understand, but what is going on with pins 3 and 4 and what is the symbol on the right that they are connected to?

Switches don't have polarity. They are either open or closed depending on whether the switch is on or off. That is to say... they either let current/voltage through or they don't. It doesn't matter which direction the current flows. Most tactile switches have 2 sets of legs, and each set is actually electrically connected to each other. It's usually only necessary to use 1 pin from each. You can use a voltmeter to test the pins to find out which ones are connected to each other. This is of course assuming a single pole single throw type switch. Actually it's right there on the diagram. Pins 3 & 4 should go to chassis ground as they protect the case of the switch. Pins 1 & 2 are what is being switched.

True enough about the polarity, I just didn't know how to refer to the different sides of the switch. I am accustomed to the usual two pairs of legs on a standard tactile switch and knowing which pair is on one side of the switch and which pair is on the other is important. I mistakenly referred to them as "positive" and "negative".

I figured that symbol might refer to ground, but I have never heard of "chassis ground" before today. Is there much practical difference between standard ground and chassis ground?