A 13RM tattoo cartridge refers to a round magnum cartridge with 13 needles arranged in a circular configuration. The "RM" in the name stands for "round magnum," which is a type of tattoo needle that is used for shading and filling in larger areas of a tattoo design, particularly when a softer or more blended look is desired. The number 13 indicates the number of needles in the cartridge. The 13RM cartridge is a popular choice for shading and blending in larger areas of a tattoo design, and it can be used on various parts of the body. It is important for tattoo artists to choose the appropriate needle configuration and size based on the design and area of the body being tattooed to achieve the desired results.