This is a Mitsubishi Uni-ball One gel pen refill, model UMR-38S, in red. With a 0.38mm ball diameter, it allows for precise writing and is a consumable item that can be easily replaced when the ink of your existing Uni-ball One pen runs out. As a genuine refill made in Japan, it is sold individually, allowing you to economically purchase the quantity you need to extend the life of your writing instrument.