I'm having some trouble fitting a model for differences-in-differences regression in Stata. Specifically, I'm confused as to the `B3` coefficient in the below:

    y = B0 + B1p + B2trt + B3(p*trt) + u

Here, `p` and `trt` are dummy variables where `p = 0` for pre-treatment and `1` for post-treatment, and `trt = 0` for control group and `1` for treatment group. `p * trt` is an interaction term indicating the marginal impact of the treatment group over the experimental period. 

My question is whether this should be a simple dummy variable equal to zero if either `p / trt = 0` and `1` if both `p / trt = 1`?