Partial plan space search uses a Backward inference search from the goal.
Each node of the search space is a partial plan
It establishes diferent types of constraints, for example, precedence constraint (a must precede b), binding constraints (inequality constraints, e.g., v1 ≠ v2 or v ≠ c or equality constraints (e.g., v1 = v2 or v = c) and causal link (use action a to establish the precondition p needed by action b).
It makes more and more refinements, until it has a solution.
POPLAN, TOPLAN and SATPlan.