A score function is a function of data and parameters whose equation or expectation characterizes an estimator or causal parameter. In orthogonalization, the causal effect appears as the solution to an equation formulated in relation to a score function. Each combination of a DAG and causal parameter has its own score function. section 8.1.3
If machine learning is familiar, think of a score function as similar to a loss function: it supplies the criterion connected to the target causal parameter. Nuisance functions, such as the propensity score, support estimation without being the primary object of interest. section 8.1.3
