Determining what "E" in a number (for example, 2.51E-13) means on the HP33S

Exponent of ten (for example, 2.51x10-13). Scientific and engineering notation express the number as a mantissa multiplied by a power of 10. The letter E separates the exponent from the mantissa.

Scientific Notation (SCI)

Scientific notation uses a mantissa with one digit to the left of the decimal point. Figure 1 above shows the example of SCI 6.   To specify scientific notation:  Press {LeftShift}, then {DISP}. Choose SC. Enter number of places up to 11. For 10 or 11 places enter [.] 0 or [.] 1.

Engineering Notation (ENG)

Engineering notation expresses a number as a mantissa with one, two, or three digits to the left of the decimal point, multiplied by 10 raised to a power that is a multiple of 3. Figure 2 above shows the example of ENG 4.  To specify engineering notation: Press {LeftShift}, then {DISP}. Choose EN.  Enter number of places up to 11. For 10 or 11 places enter [.] 0 or [.] 1.