[morzh]

As other have said,

There is only one safety capacitor rating, and it is the Y-type.

The X-type is typically is placed across the line. It is not guaranteed to fail either open or short. The formal definition of usage of that cap is "where failure of the capacitor will not lead to the danger of electrical shock".
When X-type fails, it does not expose anyone to the electrical shock. If it fails open, the filtering function of it will not work, and if it fails short, it will simply blow the fuse.

The Y-type is a true safetyy device, as it is guaranteed to fail open, and therefore the failure of it will not expose anyone to the danger of electrical shock. This is why it is placed between the line and the chassis ground (touchable by humans).
The formal definition of usage of Y-caps is:"where failure of the capacitor may involve the danger of electrical shock".
Selecting the value of the Y-cap is a compromise; the value is selected such that, while not compromising the filtering function, it does not expose a human to the so called "touch" current, where, if the GND connection were to fail, and a human were to complete the circuit to GND, the current, defined by the AC voltage amplitude and the value of the capacitor, would be below the dangerous level. Which means, that the value willl depend on the AC voltage and the frequency.

One might ask, why X-caps are needed at all, if they are not safety caps? The answer is, the X and Y-rated caps are tested against performance in AC lines, which exposes the capacitor to the AC noise, which includes many various transients with high energy content. X-cap will survive and possibly even continue to work after it experienced some damage, where a regular cap will burn.
On my working table at my company, I have two X-caps with blown off piece of shell and visible damage to the structure, which are still working fine when tested, and exhibit the nominal value when measured. These were a part of TDK plug-in 12V power supply, which went during some very rigorous immunity test; when I opened it (it is not repairable, but can be opened), the fuse was gone, but the rest of the circuitry was OK.

And, the last:

in the old equipment the X/Y caps fulfilled different function than the one they are used for today: back then they protected the device (the radio) FROM the noise, carried by AC lines.
Today they protect the AC lines against the noise generated by the device.
This happened with the advent of switching power supplies.