Selection parameters of a X & Y rated capacitors
X and Y type capacitors (as it turns out) are types of safety capacitors, usually for higher voltage. (The X and Y letter codes can also mean a low temperature coefficient for class 2 ceramic capacitors.)

Here are the relevant points from a Safety Capacitor Basics document from Tecate Group:


X Capacitors are also known as "across the line" capacitors. They are used between the "live" wires carrying incoming AC current. They're used in applications where failure of the capacitor will not lead to risk of electrical shock to the user.
Y Capacitors are also known as "line to ground" capacitors. They are used in applications where failure of the capacitor could lead to the danger of electrical shock to the user, if the ground connection is lost.