12mm is from bottom of head to end of screw. Basically the threaded part.
Don't worry about running out of a screw size as you can get these very easily in most hardware stores. What country do you live in? Pleae update your profile.
99.9% of the screws and nuts in the UMO are 3mm, standard thread (I think standard is .5mm per thread) (versus extra fine thread which is harder to find in stores). Also almost everything has a 2mm hex head. I'm in the USA and they are found in the metric section and usually called "M3" screws (metric 3mm) and they are also called "button head screws" versus countersunk, round head, flat head, etc.
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IRobertI 516
1203 is just Ultimaker's internal part number.
The measurement is the threaded part only.
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