The level dividing rules and classifying principles of engineering semantics of the practical engineering drawings are advanced for the first time. According to the process of the projection expression

the semantics are divided into expression mode semantics

projection relation semantics and dimension constraint semantics. Furthermore

every level is classified to lower explicit semantics and upper hidden semantics

which the engineering semantics is characterized by dynamic

hierarchy and formalization through analyzing systematic the explicit semantics and digging deepening the hidden semantics.