It cannot be packaged or transported by another vehicle. As such, it must be driven to its destination, regardless of where it is.
The BMS - Ironship is a large logistical ship specially designed to transport large quantities of equipment and supplies from backline logistical towns (also known as 'Logi Hubs') to midline Seaports, where its cargo can be unloaded, and either stored in them, or transported to the frontlines. While freighters are supposed to be unloaded at Seaports, they can be unloaded anywhere, as long as there is a nearby crane in order to pick up the contents inside of it.
The ship itself has an inventory that can hold some items, but its true strength comes with the large amount of shippables it can carry. Loading and unloading the cargo is done much more easily at a Seaport because it has Stationary Cranes and can store all shippables.
While long distance deliveries from Seaport to Seaport are the primary role of the freighter, it can be used in resource collections, by bringing gathered resources from Resource Fields to Seaports (cargo full of Resource Containers that are then transported to a Refinery by Flatbed)
Due to their size and durability, freighters are often used to block Bridges and narrow waterways, to prevent access by landing fleets and partisans.
Prior to Update 0.30, the BMS - Ironship didn't have a Passenger seat.
Prior to Update 0.46 ('Entrenched'), the BMS - Ironship had to be researched in order to be constructed. This was later changed in order to improve logistics capabilities, especially during early-game.
Prior to Update 1.54 ('Naval Warfare'), the BMS - Ironship was the largest Naval Vehicle in the game. This changed with the introduction to Large Ships, which drastically changed naval combat and logistics, especially during late-game.
The BMS - Ironship from the front
The BMS - Ironship from the rear
A BMS - Ironship with a Flood Mk. I loaded inside of it
Multiple freighters loaded by a Crane with packaged Containers
The cog symbol appearing when a crane can store a shippable in the open cargo hold