The Diesel Oxidation Catalyst (DOC) and Diesel Particulate Filter (DPF) segmentation by type reflects the fundamental engineering choices that drive cost, durability, and performance. DOCs are chiefly classified into monolithic substrates, coated washcoat structures, and advanced nano‑engineered formats. Monolithic units dominate the market due to their low pressure drop and established manufacturing base, accounting for roughly 60% of DOC volume. Washcoat variants, which embed catalytic metals on high‑surface‑area supports, capture about 30% by offering higher conversion efficiency for CO and HC emissions. Nano‑engineered catalysts, still emerging, hold the remaining 10% but are projected to grow as stricter emission standards push for higher activity per unit volume. For DPFs, the primary type categories are wall‑flow (cordierite), silicon‑carbide, and metal‑foam designs. Wall‑flow filters remain the workhorse, representing 55% of installations because of their balance of filtration efficiency and thermal resilience. Silicon‑carbide filters, prized for superior heat‑tolerance, comprise 35%, while metal‑foam solutions, optimized for low back‑pressure, occupy the final 10%.