The IP (Ingress Protection) rating given to a piece of electrical apparatus is a two digit code indicating the degree of protection its enclosure affords it. The first digit represents protection against penetration by solid objects accessing hazardous parts, the second describing the enclosures, protection against the ingress of water. An X in place of either digit means that either the enclosure has not been tested or that the test is not applicable.
In the event of additional holes being drilled, pierced or knockouts removed, suitable measures should be taken to restore the product to its original rating IEC 529, BS EN 60529 does not apply to protection against the risk of explosion or conditions such as humidity, corrosive gases, fungi or vermin. In certain cases equipment designed to be mounted in an enclosure will contribute towards the stated IP rating, (e.g. pushbuttons mounted in an enclosure). Different parts of enclosures can have different degrees of protection and still conform to the standard.
Source: TLC direct.