Definition
Travel is necessary to move between disconnected parts of a layer, but the nozzle may ooze while pressure remains in the hotend. Slicers therefore combine travel planning with retraction, wipe, Z-hop and path-avoidance settings.
The command letter alone is not enough to identify travel. Many slicers use G1 for both printing and travel, so software must evaluate the E change under the active M82/M83 mode. A move with unchanged E is travel; a negative E-only move is retraction rather than spatial travel.
What matters in practice
Visible risk
Long travels across open areas can correlate with stringing or scars, but the path alone cannot prove a defect.
Speed matters
Travel feed rates are often higher than printing feed rates and should be reported separately.
Viewer value
Showing travel separately makes unexpected crossings and parking moves easier to notice.
Example in G-code
Print, retract, then travel
M83
G1 X40 Y20 E1.2 F1800
G1 E-0.8 F2400
G0 X90 Y70 F9000The last line changes position without positive E, so it is a travel move.
What this viewer can show
The viewer colors travel separately and lets you hide or show it while inspecting a 3D-printing file.
Primary sources
This explanation uses firmware documentation and details visible in G-code. Your printer's settings still determine what the machine does.