When referring to weatherproof junction boxes, typically we are referring to NEMA or UL standard Type 3 and above. Most commonly the NEMA or UL Type 3R Junction Box or Type 4X Junction box are used. These are the most commonly manufactured for a number of reasons. They both provide protection against contact with enclosed equipment as well as protection from rain, sleet, ice.
Both the Type 3R and Type 4X are ideal for outdoor use. One of the difference between the two comes down to finishing and ventilation. They are both formed from continuously seem welded steel (Steel gauge 12, 14 or 16 depending on size), the type 3r is finished with galvanized steel (G90) and is commonly powder coated for a longer lasting finish. The Type 4x is formed the same but is enclosed with type 304 stainless steel for a long lasting corrosion resistant finish.
Both utilize gaskets on the covers to seal out water, ice and rain. The 4x adds an extra layer of protection by being able to stand up to hose directed water and a longer lasting finish. The type 3R has ventilation louvers so is not ideal for dusty situations, however the Type 3RX and Type 4 and Type 4x are.
Both are great weatherproof junction boxes and will get the job done both indoor and outdoor.