This read-only variable will return true if the instance is classed as being a kinematic object, or false if it is not. A kinematic instance is one that has infinite mass (a density of 0) but can move. So, to make an instance kinematic, you would first create a static instance and then set one or more of the instance variables related to movement (ie: phy_speed_x)
Boolean (or undefined if the instance is not physics enabled)
phy_speed_x = 5;
The above code checks to see if the instance is kinematic and if it is not, it sets the horizontal speed to 5.