Final answer:
The second letter of the orange book designation refers to the type of orbital, such as 's', 'p', 'd', or 'f', which indicates the shape and energy level of the subshell where electrons are located in an atom.
Step-by-step explanation:
The second letter of the orange book designation likely refers to the letter that designates the orbital type (the subshell) in electron configuration notation in chemistry. For example, in the electron configuration of carbon (C), which is 1s22s22p2, the first letter 's' refers to the s orbital subshell of the first energy level, and the second letter 's' refers to the s orbital subshell of the second energy level. The 'p' that follows is the orbital type for that subshell. In this context, the letters 's', 'p', 'd', and 'f' correspond to different types of atomic orbitals that electrons can occupy, and each type has a characteristic shape and energy level.