on saleDiplomat Nexus Demo Fountain Pen - Black/Silver

BLUF: The Nexus is a pen designed from the start to have the barrel filled with an eye dropper or ink syringe; cartridge/converters cannot be used. The nominal ink capacity is ~3.7ml, about the same as 4 standard short international cartridges. With that much ink capacity, you might forget that even the biggest tank has to be refilled eventually, and an ink window (or crystal clear demonstrator barrel) will let you check ink level at a glance. The standard nib is a JoWo #6 steel nib; a JoWo #6 14kt nib is available for a considerable up charge. Both types of nib are very smooth, neither of them is advertised as a “flex” nib, but the 14kt nib has a little more bounce. The ink chamber has an “automatic” shut-off valve which is closed/opened by capping/uncapping the pen, respectively.
Great writing pen, lots of ink capacity, manufactured with traditional German engineering and know-how.
TLDR: The most likely reason for barrel-filling a fountain pen is to get the most ink possible into the pen. Cartridges, converters, piston and vacuum mechanisms take up space where ink can’t go, and simply filling the barrel with ink will double or triple your ink capacity, but there’s a price to pay. Almost any pen can be a barrel filler pen once, as long as the ink window is covered by clear plastic (Diplomat Magnum, Lamy Safari, Lamy AL Star, etc., have two uncovered holes in the side of the barrel as ink windows), but most pens need some “hacks” to make sure the ink stays where you want it to stay. And some inks’ components may corrode metal, so…just because you CAN doesn’t mean you SHOULD.
Simple barrel filler pen designs just have an empty void where you use an eye dropper or syringe to fill the barrel, screw on the grip section, and write. As long as there’s an O-ring and/or silicone grease on the threads between barrel and grip section, you’ll be okay. But as you write, the ink in the barrel is gradually replaced by air…which ALWAYS obeys Avogadro’s laws about the volume, temperature, and pressure of a gas in a specific volume of air. In short, if the air is heated, like by your hand, or sunlight, the pressure will rise. The pressure wants to get out, and the only way out is through the feed, which is full of ink. The more air, the higher the volume of air that wants to escape. If it happens while you’re writing, you can use a conveniently placed tissue paper to absorb the ink blob before it hits the paper, hopefully. If it happen while the pen is capped, well…there might be a messy surprise waiting for you when you uncap the pen.
So-called “Japanese Eye Dropper” pens use a plug to seal off the ink chamber from the feed. It’s basically like a vac-filler without the pump diaphragm. Open the valve (with the nib pointing up to minimize the mess if there is built-up air pressure in the ink chamber), leave the valve open, and write. When you’re done, point the nib up, tap the pen gently to persuade the ink in the feed to return to the ink chamber, close the valve, cap the pen, and you’re done. It’s a little more complicated than just “remove cap, write, replace cap”, but that’s the price you’ll pay if you want 4x the ink capacity of a standard cartridge. The more air is in the ink chamber, the messier it can get!
The Nexus uses a similar system, in principle. However, instead of the valve moving towards the back of the feed channel, the Nexus moves the grip section to the fixed valve/plug. The nib actually moves in and out, is spring loaded to move out and open the valve moving towards, and is pushed in by the cap being screwed back on. As long as you always point the nib up BEFORE removing the cap, and always point it up and let most of the ink drain from the feed back into the ink chamber BEFORE putting the cap back on, you’ll most likely never need those tissues sitting on your desk. Or, you could ignore everything I said and take your chances.
FWIW, EVERY fountain pen has this problem, but with most small cartridges, the air volume is so small that heating the air from 68°F to body temperature is probably not noticeable, especially if you start writing almost as soon as you pick up the pen. The bigger the air volume, the more spectacular it could become.

Awesome pen to travel with.