From ROI Wiki

The Hornblower is a class of USF starships built and dedicated entirely to the exercise of commerce, shipping and trade, and travels exclusively between Earth and the Society Joint Commerce Station (JCS). They are used to train Earth in the other side of galactic life, which is interstellar commerce. The Hornblower class is easily 3x's bigger than its sister class, the Aquarius, and is cavernous inside allowing for the shipping of everything from boxed freight to raw ores. The inside is also highly customization with the ability to add, remove, and move around decks and walls to separate cargo or allow for sections to be pressurized whenever required. If it is not required, the main hold is kept in an unpressurized state to reduce stress on the outer hull during long voyages. If only small amounts of the hold need to be pressurized, then individual pressure vessels can be loaded into the hold to allow items that require pressurization to be shipped without pressurizing the entire hold. Hornblower ships regularly make stops at Pluto Station, the JCS, Saturn Shipyards, and the Mars Colonies, as well as several other locations. They also periodically link up with Earthfleet starships on active patrol to resupply them.

This isn't necessary for them to do, as Earthfleet ships operating within four to five light years of Earth can easily resupply by portal transporter. However, it does make good practice for the crews of the Hornblower class, and so it is used periodically by Earthfleet, even though better methods of resupply are available. This is also done with the forethought that, at some point in the future, similar ships from Earth may be required to resupply Earthfleet vessels operating deep within the galaxy outside of Sol Space. So practicing this now will potentially pay dividends later on. An average Hornblower cargo ship sports a standard crew of just twenty nine people, counting crew and instructors, but can operate effectively with a crew as few as three, or as many as one hundred. As far as the internal technology goes, the Hornblower is similarly equipped to the Aquarius class, but only sports a top speed of one half light speed due to its greater size and mass.