MakerBot Industries

OK, replicator G is the carving program that creates the gcode and sends it to the bot. You are limited to BASIC scaling only and simple orientation changes to the 3D point cloud file (STL). Skeinforge is the actual open source program embedded in Rep-G that does the carving and Gcode generation.

So, any object, you open in rep-g, then hit generate gcode, and then use the control to print. If you resize the object, you then hit generate again to create new gcode and then print that gcode. Once an object has had gcode created, you can just print the gcode again for mutiple copies.

One note is critical that lets say you try rep-g before you get your bot. Ensure you select the driver for your bot before generating gcode, that you might attempt to print later once you got your bot, otherwise, the gcode will be incorrect and likely cause a problem if you tried to print it. Not sure if the latest rep-g has the driver for replicator.

So again:
STL files are point clouds and are not directly printable on any bot
Rep-G uses the configuration and driver file to carve the STL into layers and create gcode specific to the bot
Rep-g also controls and sends that code to the bot for printing

Gcode is human readable text and YES, you can edit it (not for the novice).
Rep-g can open either an STL or a Gcode file.
Note that changes to the gcode are not then represented in the image in Rep-g!
You also cannot open a gcode file and save it as an STL.
One other note is that if you save to disk(SD card) then it saves in the Binary S3G format which is not human readable text-only binary data for easier use by the firmware on the Arduino which is the bot's cpu. I also believe when you hit print-rep-g is using the same binary format rather than the human readable gcode across the usb cable.

STL > gcode > "print" / save as S3G > print
You CANNOT go s3G > anything, nor can you gcode > stl