See Market Normal Vol .
The theoretical value is the mid-rate value of the deal or portfolio as calculated using the Black model based solely on the ATM Volatility.
This value can be compared to the Market Price, which is calculated using the SD model.
In the Single Option page for a swaption and swaption strategies, by default the system displays this result for the spot premium convention. However, you can choose to toggle between the results calculated for the forward premium and the spot premium using the Premium Spot <> FWD buttons. For more information see Working With the Forward Premium Conventions.