This standard describes a system for classifying the degrees of protection provided by the enclosures of electrical equipment.
The complete standard is not available for free download, so herein I rely on secondary sources for more information, such as Wikipedia and DSM&T.
The general form of the marking or code is to be "IPmn" where "m" is a digit that represents protection against intrusion of dust into the device and "n" a digit the represents protection against water. A possible coding might be IP54. If a device is not rated for one of the elements, an X is used instead of a digit. We often see marine electronic devices rated as IPXn, with digits 0 to 9, and, for example, a device might be rated "IPX6."
The ten levels of water intrusion protection are shown below according to secondary sources. Note that attributes accumulate only in levels 1 to 6, that is, a device rated IPX7 does not necessarily also possess all the attributes of an IPX6-rated device. Here is one secondary source's description of the rating requirements:
Code - Protection
IPX0 = none
IPX1 = dripping water
IPX2 = dripping water when tilted 15-degree
IPX3 = spraying water
IPX4 = splashing water
IPX5 = water jets
IPX6 = powerful water jets
IPX7 = immersion up to 1-meter depth
IPX8 = immersion greater than 1-meter depth
IPX9 = powerful high-temperature water jets
More details at: https://en.wikipedia.org/wiki/IP_Code#Second_digit:_Liquid_ingress_protection
For each rating code there is also a minimum duration of the testing specified. A common rating is IPX8, and the manufacturer usually specifies the duration and depth of the testing. For example, a GX1300 radio from Standard Horizon is rated IPX8 with the notation "1.5M or 4.92 feet for 30 minutes" to clarify the test depth and duration.
A rating of IPX7 or higher does not mean the device is also rated for protection against water intrusion from any of the elements in codes 1 to 6. For example, powerful water jets (IPX6's test) might cause water intrusion in an IPX7 rated device