What is an X2 rated capacitor?

What is an X2 rated capacitor? Ie a 220v Cap X2 rated means? 440V?

X2-MKP METALLIZED POLYPROPYLENE FILM CAPACITOR,CLASS X2

INTRODUCTION
Are non-inductively wound with meatllized Polypropylene film as the dielectric/electrode with copper-clad steel leads and encapsulated in a plastic case sealed with epoxy resin.

APPLICATIONS
As an across_the-line type noise suppression capacitor,and suitable for AC purpose
Line-By-Pass,Antenna coupling,

FEATURES
Box type provides the identical outer appearance
Very low loss at high frequency, suitable for high current
High insulation resistance,long life due to self-healing effect
Withstanding 2.5KV impulse voltage,Class X2
High moisture-resistace.

https://www.tradekey.com/product_view/id/132975.htm

in other words, the x2 rating has to do with various safety and construction related specifics ... it doesn't mean rated voltage multiplied by two

Withstanding 2.5kV impulse voltage is the key part.