Filler (also called grist) is a petroleum- or coal-based coke fraction of a green, carbon mix or formulation. Coarse particles, > 0.425 mm, are sometimes referred to as tailings; fine particles, < 0.074 mm, are referred to as flour. graphite flour is also used as a filler.
PAC, 1995, 67, 473. 'Recommended terminology for the description of carbon as a solid (IUPAC Recommendations 1995)' on page 489 (https://doi.org/10.1351/pac199567030473)