In fact, this is contrary to the original intention of the national standard; the standard requires that engineers select a tolerance grade according to the national standard, and then check the corresponding deviation value according to the grade, because many empirical values are not within the tolerance grade of the national standard. The specific selection of the level, you can in the "Machine Design Manual" to choose.
At present, general-purpose equipment (conveying machinery, lifting machinery, packaging machinery, etc.) commonly used in the economic processing tolerance of 7, a little more precise for the 6 level. Regardless of what machinery, we must be able to ensure the accuracy and interchangeability of parts, but also to ensure the economy of manufacturing equipment, in general terms, that is, if a lower level of accuracy (such as 7, 8) to ensure that the function of the machine and accuracy, do not ask for parts of the accuracy of the overly high. That will increase the manufacturing costs and operating difficulties, as well as the choice of equipment. Because the higher the precision requirements, the higher the choice of processing machine tool requirements.
To summarize: seven, is very affordable, and can ensure that most of the requirements of the use of the grade. Aerospace, military, medical, and other industries need to follow the industry standards.