Definition
A slicer chooses Z positions for each layer based on the selected profile and geometry. Smaller layers can reproduce finer vertical detail but usually increase the number of passes and print time. The first layer may intentionally be thicker than later layers to improve tolerance and adhesion.
Layer height is not always one constant saved in the file summary. Variable-layer slicing changes the Z step through the model, vase mode may raise Z continuously, and sequential printing can return to a lower Z position for another object. A careful viewer separates values saved by the slicer from heights measured in the movement commands.
What matters in practice
Nominal vs measured
A profile value describes intent; Z moves reveal what the file actually commands.
First layer
The initial print height is often configured separately from normal layers.
Not the nozzle diameter
Layer height and nozzle diameter are related process choices but are not the same dimension.
Example in G-code
Two 0.2 mm layers
G1 Z0.20 F600
G1 X40 E1.2 F1800
G1 Z0.40 F600
G1 X0 E1.2 F1800The extrusion paths occur at Z0.20 and Z0.40, giving a 0.20 mm increment in this simple example.
What this viewer can show
The viewer compares layer and height values saved by the slicer with values found in the movement commands. Different calculation methods can produce different results, especially for complex prints.
Primary sources
This explanation uses firmware documentation and details visible in G-code. Your printer's settings still determine what the machine does.