Lead = Its actually Graphite.
We all use HB pencils. HB=Hard Black.
This is the mid range of pencil hardness/blacknes. The softer the lead, the blacker the pencil, the harder the lead the lighter the grey.
Generally soft range leads: 4B, 3B, 2B and B are usually found in an Artists tool kit, while B, HB, H, 2H, 3H and 4H are Draftsperson's tools. Soft leads are smudgy, lending themselves more to Art. It isn't cool to have smudgy draftings.
Line weight refers to the thickness of the line. Because 4H pencils can be kept the sharpest, and because they are the lightest, 4H would typically be used for .03-5 light construction lines. 3H, a shade dark enough to solidly print, yet still hard enough becomes the .05 dimenson lines and the note leaders. Some hatches are effective in 3H. 2H is for overall detail object lines, hatch, preliminary draft; .05 - .07. H is for hard object lines (openings), notes and render shading. .07-.09. HB should only be used selectively as it smudges. I use it for ground lines, overall object lines and titles.
Most Draftspersons operate on a 3 pencil range. Less will never look professional.
Before mechanical pencils were invented the Draftsperson had to create the line weights by hand using hard/soft pencil techniques. The skill level a Draftsperson had in mastering line weights largely defined their success.