This is felt tipped or other porous-tipped pens and markers.
Copic Markers were intended for use by illustrators; their soft colours and brush-like tips make them ideal for colouring and blending. But hobbyists are opportunists and Copic Markers are now the fashionable means of finishing your model kits -- mecha and figures alike are being shaded and highlighted with Copic Markers! But this "Warm Gray" pen with a 0.03 mm tip (extremely fine!) was devised just for plastic kits, and is perfect for outlining panel lines on your mecha, with a much more realistic, soft shadow than a plain black line would give you!