I'm trying to create a part for Fritzing and I am struggling to interpret the packaging size in the datasheet. 
In the notes it is said that the sizes are in mm, ok with that, but how should these numbers be interpreted? A division or max/min does not make sense.
Answer
As Nick says, it's the maximum and minimum value that dimension can be.
In your example the width of the chip can be between 2.9mm and 3.1mm. Some datasheets would instead put "3mm ±0.1mm" which amounts to the same thing.
To get the nominal value you can subtract the lowest from the highest, divide by two, then subtract the result from the highest value.
- 3.1mm - 2.9mm = 0.2mm
- 0.2mm ÷ 2 = 0.1mm
- 3.1mm - 0.1mm = 3mm
No manufacturing system is perfect, and you have to allow certain tolerances in your design to allow for these inaccuracies.
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