In the 49 CFR 172.101 Hazardous Materials Table, what does the symbol "+" in Column 1 mean for an entry?

AAMVA CDL Manual, Section 9.3.3 (nhcdm.pdf p. 9-4); 49 CFR 172.101(b)

Misreading a Column 1 symbol means shipping a material under the wrong name, class, or packing group — which cascades into wrong placards, wrong packaging, and wrong emergency response information. Drivers and shippers both rely on these symbols to read the table correctly the first time.