AssembliesComponents that aggregate several joints for analytic loop handling 
Universal  prismatic  spherical joint aggregation (no constraints, no potential states) 

Universal  spherical  revolute joint aggregation (no constraints, no potential states) 

Universal  spherical  prismatic joint aggregation (no constraints, no potential states) 

Spherical  spherical  revolute joint aggregation with mass (no constraints, no potential states) 

Spherical  spherical  prismatic joint aggregation with mass (no constraints, no potential states) 

Planar revolute  revolute  revolute joint aggregation (no constraints, no potential states) 

Planar revolute  revolute  prismatic joint aggregation (no constraints, no potential states) 
This information is part of the Modelica Standard Library maintained by the Modelica Association.
The joints in this package are mainly designed to be used in kinematic loop structures. Every component consists of 3 elementary joints. These joints are combined in such a way that the kinematics of the 3 joints between frame_a and frame_b are computed from the movement of frame_a and frame_b, i.e., there are no constraints between frame_a and frame_b. This requires to solve a nonlinear system of equations which is performed analytically (i.e., when a mathematical solution exists, it is computed efficiently and reliably). A detailed description how to use these joints is provided in MultiBody.UsersGuide.Tutorial.LoopStructures.AnalyticLoopHandling.
The assembly joints in this package are named JointXYZ where XYZ are the first letters of the elementary joints used in the component, in particular:
P  Prismatic joint 
R  Revolute joint 
S  Spherical joint 
U  Universal joint 
For example, JointUSR is an assembly joint consisting of a universal, a spherical and a revolute joint.
This package contains the following models:
Model  Description 

JointUPS  Universal  prismatic  spherical joint aggregation 
JointUSR  Universal  spherical  revolute joint aggregation 
JointUSP  Universal  spherical  prismatic joint aggregation 
JointSSR  Spherical  spherical  revolute joint aggregation
with an optional mass point at the rod connecting
the two spherical joints 
JointSSP  Spherical  spherical  prismatic joint aggregation
with an optional mass point at the rod connecting
the two spherical joints 
JointRRR  Revolute  revolute  revolute joint aggregation for planar loops 
JointRRP  Revolute  revolute  prismatic joint aggregation for planar loops 
Note, no component of this package has potential states, since the components are designed in such a way that the generalized coordinates of the used elementary joints are computed from the frame_a and frame_b coordinates. Still, it is possible to use the components in a tree structure. In this case states are selected from bodies that are connected to the frame_a or frame_b side of the component. In most cases this gives a less efficient solution, as if elementary joints of package Modelica.Mechanics.MultiBody.Joints would be used directly.
The analytic handling of kinematic loops by using joint aggregations with 6 degrees of freedom as provided in this package, is a new methodology. It is based on a more general method for solving nonlinear equations of kinematic loops developed by Woernle and Hiller. An automatic application of this more general method is difficult, and a manual application is only suited for specialists in this field. The method introduced here is a compromise: It can be quite easily applied by an end user, but for a smaller class of kinematic loops. The method of the "characteristic pair of joints" from Woernle and Hiller is described in: