This photo shows drafting and clutch pencils. The drafting pencils are on the right. They come in .03, .05, .07 and .09 mm sizes (the size of the hole at the tip). They’re filled with sticks of graphite that come in different degrees of hard and softness (light and dark). It’s good practice to keep a .07 filled with a dark lead (such as HB) and a .05 filled with a light lead (such as 4H), to maintain a strong contrast between object and construction lines. Clutch holders are for 2 mm size leads, which are sharpened in a lead pointer (stick the pencil in it and rotate) or by rubbing on a sandpaper block. While a greater variety of pencil strokes can be obtained with them, the constant sharpening they require is tedious. The smaller drafting pencils work just fine. Don’t confuse them with the very cheap mechanical pencils bought in a pack for a dollar, though. The difference is in the design at the tip. You will get better drafting results with a bona fide drafting pencil.
Read more in my book, Architectural Drafting for Interior Designers.