Yeah it's probably field specific and I guess Gaussian-based uncertainty would be more about statistical sampling rather than tolerances. I've noticed that if arithmetic is being done on it it's almost certainly Gaussian. I just mean whenever I see uncertainty like this, I don't know what is meant!

In Mechanical Engineering, tolerances ensure that when you put parts together, they will fit as long as the tolerances were respected.

It's not statistical. If the machinist makes a part that's not within the +/- bounds, they throw it away and start again. If you tried to fit multiple parts, all with only statistical respect for tolerances, you would run into trouble almost 100% of the time with just a few pieces.