Calculating Combined Measurement Uncertainty - VDA 5




I am very new in this forum and now learning about Measurement Uncertainty using VDA5 Quality Management in the Automotive Industry - Capability of Measurement Process Manual.
In this manual, in order to figure out the standard uncertainty of the measurement system, standard uncertainties of components, such as u-re, u-cal, u-evr, etc, must be calculated.
Then when calculating the combined measurement uncertainties, the following equation is applied: (VDA5 P68, Table 12)

u-ms = sqrt( u-cal^2 + max(u-evr^2, u-re^2) + u-bi^2 + u-lin^2 + u-msrest^2)

My question is that why the greater value of u-evr and u-re is used.
there is a brief explanation about this in the page 56. But I do not quite understand it.
Since I am a new learner about this topic, any information will be appreciated.
If anyone know the web site I should visit, please let me know.
Thank you.
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