opacity
Opacity is how much of what is already on the page a mark hides, said as a number from zero to one.
In more detail
One is ordinary paint: what is drawn covers what was there before it, the way a sticker covers a table. A half lets half of the older colour come back through. Zero paints nothing at all. Everything in between is a proportion, and nobody has to work out the colour that results: whatever draws the page does that as it draws.
There are two of these numbers, not one. Filling a shape and drawing the line round its edge each carry their own, so a box can be a pale wash inside with a solid frame around it. Both are stated once, in a small block of settings the page turns on by name, and they hold for everything drawn afterwards until the page turns them off again.
What it does on a real document
A word laid across a whole page at a fifth of its strength leaves every line under it readable — that is the whole of how a mark of that kind is made, and there is nothing to prepare in advance. The same number turns a plain rectangle into a highlight over a paragraph, and a black bar into the grey band a heading sits on.
A photograph can bring its own, one value for every dot in it. A logo cut out of its background arrives with a record of which of its dots are see-through and by how much, and that record travels into the document unchanged, so the corners of the logo do not come back as a white square.
Going to a press, all of it is worked out and redrawn solid at the last moment, on the printing works' own equipment. What you see on the sheet is the colour the mixing produced, not the two colours that produced it.
Where it comes from, and why
The format had none of this until 2001. A page was marks in order, each one hiding whatever it landed on, and a designer who wanted a fade had to send a photograph of one — worked out somewhere else, at one size, and impossible to change afterwards.
The numbers live in a named block of settings rather than beside each shape, which is not an accident: one block serves a hundred marks, and a page that would otherwise repeat the same two numbers a hundred times states them once.
See it on a finished document
Transparency and blend modes One colour, seen through four stencils