The other two insertions are indica

The other two insertions are indicated by the following convention: Sijk indicates a reading performed with the current leads connected to the line “i” and the voltmeter leads connected between the phases “j” and “k.”
If “i” is equal to “j”, one is omitted (e.g., the notation P12 (b in Figure 3.17)).
The active power absorbed by a single phase is usually referred to as P1.