Random lines are being printed? I recently got a Creality Ender-3, and tried printing a few things for some tests. I’ve printed a cube and just printed a cylindrical tube today, and I notice each time, it adds this random line on the left and a sort of outline around the actual print. Neither of these were there in my Cura file, but they’re always printed and I’m not sure why?
If the printer is printing, it is instructed to do so by the G-code file unless you are printing through an external software program that has extra G-code to print before your print starts. E.g. in OctoPrint print server it is possible to execute G-code before the print starts.
Left line = Priming
The straight line on the left is typically used to prime the printer nozzle to get the filament flow starting, this is typically seen in PrusaSlicer (Prusa's fork of Open Source toolpath generator for 3D printers Slic3r). This straight line is called priming line, purge line or intro line, and is typically (but not necessarily) printed outside or at the edge of the bed area. Furthermore, a prime line print routine will catch errant nozzle ooze, test extrusion (it is the first indication if the nozzle to bed distance is correct; if not you can abort with minimal material loss) and perform a final wipe action to avoid stringing between the prime line and start of the print. Note that this straight prime line is not a standard option in a custom profile of Cura, so this was part of the Ender-3 Preset you imported or possibly you have copied a starting G-code that includes this prime line.
A typical set of G-code lines to create a prime/purge/intro line is found in your start G-code and could look similar to:

G1 Y-3.0 F1000.0 ; go outside print area
G92 E0.0
G1 X60.0 E9.0 F1000.0 ; intro line
G1 X100.0 E12.5 F1000.0 ; intro line
G92 E0.0

After slicing your object, you will find such lines in the generated G-code file, but they are not displayed in the preview. Further information can be found in Writing G-code : swiping at start of print
Equidistant line = Skirt
The lines at distance from the print object is called the &quot;skirt&quot;, the skirt is an option found under the &quot;Build Plate Adhesion&quot; options in your slicer. The function of the skirt is similar as described for the straight prime line, but it has additional effects that can be wanted. It also shows fairly fast if the bed is unleveled as a whole or if the bed is greasy. Please look into: &quot;What are main differences between rafts, skirts and brims?
&quot;.
Note that it is usually superfluous to use both the prime/purge/intro line and the skirt, both have a similar function. The benefit of the skirt is that you can configure it within the slicer (e.g. length of the printed skirt, height to use as a shield for draft or ooze and distance to product). The downside is, that a skirt limits the useable build area by the distance and width of the skirt.