What the thing looks like by day — a white round tower, a red band, a house alongside, a lattice mast. That is what you are actually looking for at two in the afternoon.
How high the structure is, as opposed to how high the light is above the sea.
How far you will actually see it. Twelve miles is the nominal range, quoted for one fixed state of the air. What you get depends on the weather and on how high your own eye is above the water, and that comes off a table in the book — not off the chart.
The exact bearings a sector light changes colour on, when the chart has only room to draw the sectors approximately.
Its international number, which is how the light is named in every other publication and in every correction.

Module 01 · Publications