Definition
First-layer reliability depends on nozzle-to-bed distance across the print area, a clean compatible surface, material temperature and appropriate motion. Slicers often use a separate first-layer height, slower speed, different line width and reduced fan to improve tolerance and adhesion.
A file can reveal the intended Z height, paths, speeds, temperatures and cooling commands. It cannot show whether the sheet is clean, the Z offset is correct, the filament is dry or the active mesh matches the bed. Those are machine and process conditions.
What matters in practice
Foundation layer
Poor adhesion or an incorrect gap propagates into shifting, warping or print failure.
Often has special settings
First-layer height and speed may differ from normal layers by design.
File is only intent
The written path cannot certify surface condition or real nozzle gap.
Example in G-code
A simple first-layer start
M190 S60
M109 S210
G1 Z0.20 F600
G1 X100 E4 F1200The file waits for temperatures, moves to a nominal first-layer Z and prints a slow line. Actual adhesion still depends on the machine.
What this viewer can show
The viewer can isolate the first layer, inspect its travel and extrusion paths, and compare file-written layer metadata with measured Z movement. It cannot judge adhesion from geometry alone.
Primary sources
This explanation uses firmware documentation and details visible in G-code. Your printer's settings still determine what the machine does.