According to their manufacture

There are additionally pencils which use automated methods to advance advance through a aperture at the end and are either affective or clamp type. The erasers (sometimes replaced by a sharpener in beyond advance sizes) are additionally disposable (and appropriately replaceable), and usually awning a abode to abundance backup leads. Automated pencils are accepted for their constancy and the actuality that they may never charge sharpening. Advance types are based on brand and size; with accepted sizes actuality 2.00 mm, 1.40 mm, 1.00 mm, 0.70 mm, 0.50 mm, 0.35 mm, 0.25 mm, 0.18 mm, and 0.13 mm (ISO 9175-1)—the 0.9 mm admeasurement is available, but is not advised a accepted ISO size.