The levels are named using an A10 string. The element code is at the beginning (e.g. HE) and the following (one or two) numbers indicate the ionization stage (e.g. HE2). This is mandatory. The next numbers indicate the principal quantum number of the level (e.g. HE26). For levels with a complicate configuration, the electron system and the magnetic quantum number may be added (e.g. O42P 2PO). Singly excited levels are marked by a “ ’ ”, doubly excited by a “ ’ ’ ” directly before the electron system indication (e.g. N33S' 2PO). In the case of explicitly considered multiplet splitting the angular momentum is added at the end (e.g. C452D5/2).