"Ah, one additional thing I'm confused about -- ..."

Ah, one additional thing I'm confused about -- what do $X_i$ and $x_i$ refer to? I thought $X_i$ referred to the node (so that SEASON would be $X_0$, {RAINING, SPRINKLER} $X_1$, {SIDEWALK} $X_2$, and {SLIPPERY} $X_3$), but then I'm not sure what lowercase $x_i$ would refer to…