Definition
A positive extrusion delta pushes filament toward the nozzle. When it accompanies XYZ motion, the slicer intends to lay down a bead; when it occurs without XYZ motion, it is usually a prime or purge. A negative delta retracts filament to reduce pressure and ooze.
E is a commanded drive distance, not a direct measurement of plastic leaving the nozzle. Flow calibration, filament diameter, drive slip, pressure compensation and clogs can all make physical output differ from the file. A viewer can measure the instruction stream but cannot certify the real bead.
What matters in practice
Two distance modes
M82 uses cumulative E positions; M83 uses per-move extrusion amounts.
Volume estimate
Filament length and diameter can estimate material volume, but slicer metadata may be more authoritative when present.
Geometry link
A positive E delta plus spatial motion is the usual signal that a path deposits material.
Example in G-code
Relative extrusion
M83
G1 X50 Y0 E2.0 F1800The toolhead moves 50 mm while the extruder advances 2 mm of input filament.
What this viewer can show
The viewer follows extrusion mode, G92 resets, retractions, and re-primes so it does not count every positive E value as new material. The result comes from the commands in the file, not a physical measurement.
Primary sources
This explanation uses firmware documentation and details visible in G-code. Your printer's settings still determine what the machine does.