The Microsoft TechNet page How Resultant Set of Policy Works begins with:

Administrators can use the Resultant Set of Policy (RSoP) snap-in for two purposes: to predict the cumulative effect of Group Policy objects (GPOs), or to determine the actual result of Group Policy settings on a particular computer, user, or user on a computer.

Although administrators can use the RSoP snap-in for reporting and planning the effects of Group Policy, much of its functionality has been subsumed into Group Policy Management Console (GPMC), which provides a much better experience for the network administrator.

In an ideal environment, administrators are encouraged to use the GPMC features for simulating Group Policy or determining the effect of Group Policy on a particular user or computer.

See How Resultant Set of Policy Works.