What do the pipe materials C1 and C2 mean?

HG/T 20519-2009 Unified Specifications for the Content and Depth of Construction Drawings in Chemical Process Design 2.0.1 Composition of the grade code: The grade code for pipe materials consists of three elements. The first element denotes the nominal pressure grade of the pipe, the second element indicates the sequence number of the pipe material grade, and the third element represents the category code of the pipe material. L 1 B 2.0.2 Regulations on pressure grade codes: 1. The first element is the nominal pressure grade code of the pipe, expressed in uppercase letters. A-G are used for ASME standard pressure grade codes, while H-Z are used for domestic standard pressure grade codes (where I, J, O, and X are not used). 2. The nominal pressure class codes according to ASME standards: A – 150LB (2 MPa); B – 300LB (5 MPa); C – 400LB; D – 600LB (11 MPa); E – 900LB (15 MPa); F – 1500LB (26 MPa); G – 2500LB (42 MPa). 3. The nominal pressure class codes according to domestic standards: H – 0.25 MPa; K – 0.6 MPa; L – 1.0 MPa; 203 – M – 1.6 MPa; N – 2.5 MPa; P – 4.0 MPa; Q – 6.4 MPa; R – 10.0 MPa; S – 16.0 MPa; T – 20.0 MPa; U – 22.0 MPa; V – 25.0 MPa; W – 32.0 MPa. 2.0.3 Specification for the sequence numbers of pipe material grades: The second digit represents the sequence number of the pipe material grade, and it is represented by Arabic numerals ranging from 1 to 9. When the pressure rating and the code for the pipe material category are the same, there can be nine different series of pipe material grades. 2.0.4 Regulations on the codes for pipeline material categories: The third section consists of the codes for pipeline material categories, which are represented by uppercase English letters. A —— Cast iron B —— Carbon steel C —— Ordinary low-alloy steel D —— Alloy steel E —— Stainless steel F —— Non-ferrous metals G —— Non-metals H —— Linings and internal anti-corrosion 3. Pipeline material grade table

So, for those 400 lb here, there isn’t much Mpa; what should it be?

400x0.454/9.81=18.51MPa