Illuminarti fully answered properly but just to clarify 2 things:
Also are the settings which i save in cura, are saved in the file on the sd card?
Yes - they are in the gcode file at the very end. Cura has a feature to load them back in under "file" "load profile from gcode".
If you increase the setting to 200%, it will print twice as fast.
Thus overriding your minimum layer time settings. For example if you said minimum layer time of 3 seconds and it is printing something tiny and you print at 200% it will probably print each layer in 1.5 seconds and you will get bad result. But for most things this is fine.
Also it's not necessarily twice as fast - the REQUESTED move speed is twice as fast but it takes a few milliseconds to get up to speed due to the current acceleration settings. Cura doesn't mess with acceleration settings - that's stored in Marlin and can be changed with the ulticontroller. And saved.
I usually set Cura to 100mm/sec and then I can set the % on the ulticontroller and now the % matches the speed. For example if 50% feed rate then it prints at 50mm/sec. That way the math is simple and I can decide on the print speed *after* I slice and put the card in.
Yes, the speed override in the controller is to adjust whatever speed is hard-coded into the gcode file. You don't always get the full speed that you requested anyway - for instance even if you tell Cura that you want to print at 60mm/s, it may slow down some or all of the print to meet a minimum layer time setting, so that the print has time to cool between layers. But whatever speed the gcode asks for, that speed gets adjusted by the percentage amount in the controller. So if you leave it at 100%, the speed is unchanged. If you increase the setting to 200%, it will print twice as fast.
The gcode file on the SD card is a set of specific instructions - telling the head exactly how to move, how fast, and how much to extrude as it does so, in order to build up the object that was sliced. It's just a text file. Each line describes one move of the head - where to move to, how fast, and how much plastic to extrude. All of the settings that you make in cura are therefore implicit in that set of instructions that gets written to the card. If you change the layer height, or many of the other parameters, then a totally different set of instructions will be needed to get the head to trace out the same shape, but using a different number of layers, each of which has to trace out a slightly different cross-sectional shape than for the original layer heights.
In addition however, Cura includes a brief description of the most important settings - such as speed and layer height - in comments at the start of the gcode file, and a compressed copy of the full settings that were used at the end of the file. This allows you to reload the settings from a previously sliced gcode file if you want to reuse or tweak those settings.
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