IP rating

The first digit is protection from solids (dust), the second from water. IP54 means limited dust and splashes from any direction, IP65 is dust-tight and protected from water jets, IP67 survives short immersion. A higher number does not mean a better robot, just a more sealed one.

For a mopping robot, a restaurant kitchen or an outdoor security patrol, this is essential. A robot with no rating should be kept dry. Also note that a rating usually covers the body, not the charging contacts or the open hand.