MSK-12C01 a simple switch
This is a SPDT (Single-Pole, Double-Throw) switch.

The way they drew the circuit symbol looks a little weird, but I've seen this on other switch datasheets. May be a new Chinese standard, I don't know.



The U-shaped outline represents the sliding metal contact, and the arrows represent the fixed contacts. So pin 2 is the common pole, and pins 1 and 3 are the two "normally open or normally closed" contacts.

Also note in the name they call it (1P2T), single-pole double-throw. Again, I don't know why they used the term 1P2T instead of the more common term SPDT.

Edit:

Here's a more detailed visualization of what this slide switch symbol means. The sliding metal contact is highlighted in yellow, and I've edited the image to show the circuit when the switch is in "1-2" position, and when the switch is in "2-3" position. Note that the common "2" is a wider contact, shown with two arrow points. It's hard to tell because there is so little information in the datasheet, but it might actually be constructed with a U shaped sliding contact, so the common really might be two contact points, connected together. That would explain why the common terminal is not in the exact center.



This is where slide switches and rocker switches behave differently: when the slide switch is slid towards the "1" contact, then common "2" is connected to "1" instead of "3". (This is pretty intuitive, but it's backwards from how a rocker toggleswitch behaves.) So in other words, if your board silkscreen has "MODE=A" next to the "1" end of the switch, and "MODE=B" at the "3" end, when the user slides the switch towards the "MODE=A" silkscreen, then points "1" and "2" are connected.