O que é HJM drift condition?

Example 8.4 (Hull–White is an HJM model)

Chapter 7’s model has $\sigma_f(t,T) = \sigma e^{-\kappa(T-t)}$: its drift is $\sigma^2e^{-\kappa(T-t)}(1-e^{-\kappa(T-t)})/\kappa$, 4.1 basis points a year for the ten-year forward with $\sigma = 80$ basis points and $\kappa = 3\%$. Volatilities of this exponential form are exactly those for which the forward curve is driven by a single Markov state; a general $\sigma_f$ makes the short rate path-dependent and the model can only be simulated.