  The brightest of all time (BOAT) GRB221009A show evidence for a narrow,
evolving MeV emission line. Here, we show that this line can be explained as
due to pair annihilation in the prompt emission region, and that its temporal
evolution is naturally explained as the high-latitude emission (emission from
higher angles from the line of sight) after prompt emission is over. We
consider both the high and low optical depth for pair production regimes, and
find acceptable solutions, with the GRB Lorentz factor $\Gamma \approx 600$ and
the emission radius $r \gtrsim 10^{16.5}$~cm. We discuss the conditions for the
appearance of such a line, and show that a unique combination of high
luminosity and Lorentz factor that is in a fairly narrow range are required for
the line detection. This explains why such an annihilation line is rarely
observed in GRBs.
