In this specific context I believe “facultad de disposición sobre la acción” refers to the plaintiff’s/claimant’s right to terminate (i.e., voluntarily dismiss) a lawsuit that he has previously filed. (See the 27,300 hits on google for “voluntary dismissal by plaintiff”).

Under Spanish civil procedure rules this can be achieved in two ways: a “renuncia a la acción” which is a waiver of action without any subsequent right to sue, resulting in a “sentencia absolutoria con autoridad de cosa juzgada” (literally, a judgment for the defendant having res judicata effects, but in practice being a dismissal with prejudice). Or, there can be a “desistimiento,” which is a plaintiff’s/claimant’s abandonment of the suit without waiving the right to bring future action (“declaración del actor abandonando el proceso sin que ello suponga renuncia a la acción”). This results in an “auto de sobreseimiento de deja imprejuzgado el fondo” (literally, an order dismissing the case without a ruling on the merits, but in practice a dismissal without prejudice).

In other respects, in Spanish civil proceedings “capacidad de configuración del debate” (also called “facultad de delimitar los términos del debate”) generally refers to the parties’ “power to determine the facts in issue” and often takes place during a type of formal case management hearing known as “audiencia previa al juicio.” I don’t think I would call this “case management power”, since case management is generally the task of the judge (in Spain, precisely during the “audiencia previa al juicio) where, among other aspects, the parties can agree as to the facts in issue.