For most barcodes this value represent the width of the narrowest bar or space, measured in pixels in the image. For DataMatrix and PDF417 it is the size of the little square modules that make up the symbol.   The smallest dimension vertical or horizontal is reported.  Best performance is obtained when the module size is between 3-10.   Smaller values require extra effort for recognition.  Modules sizes smaller than 2 are often not readable.   Module sizes above 10 represent excess resolution (for the purposes of recognition) and simply slow down the process. 

Scanners refer to this pixel value as Dots.   To obtain the physical dimensions of the feature divide this value by the resolution (in dots per inch).

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