DynamicsEnumeration to define definition of balance equations 
This information is part of the Modelica Standard Library maintained by the Modelica Association.
Enumeration to define the formulation of balance equations (to be selected via choices menu):
Dynamics.  Meaning 

DynamicFreeInitial  Dynamic balance, Initial guess value 
FixedInitial  Dynamic balance, Initial value fixed 
SteadyStateInitial  Dynamic balance, Steady state initial with guess value 
SteadyState  Steady state balance, Initial guess value 
The enumeration "Dynamics" is used for the mass, energy and momentum balance equations respectively. The exact meaning for the three balance equations is stated in the following tables:
Mass balance  
Dynamics.  Balance equation  Initial condition 
DynamicFreeInitial  no restrictions  no initial conditions 
FixedInitial  no restrictions  if Medium.singleState then no initial condition else p=p_start 
SteadyStateInitial  no restrictions  if Medium.singleState then no initial condition else der(p)=0 
SteadyState  der(m)=0  no initial conditions 
Energy balance  
Dynamics.  Balance equation  Initial condition 
DynamicFreeInitial  no restrictions  no initial conditions 
FixedInitial  no restrictions  T=T_start or h=h_start 
SteadyStateInitial  no restrictions  der(T)=0 or der(h)=0 
SteadyState  der(U)=0  no initial conditions 
Momentum balance  
Dynamics.  Balance equation  Initial condition 
DynamicFreeInitial  no restrictions  no initial conditions 
FixedInitial  no restrictions  m_flow = m_flow_start 
SteadyStateInitial  no restrictions  der(m_flow)=0 
SteadyState  der(m_flow)=0  no initial conditions 
In the tables above, the equations are given for onesubstance fluids. For multiplesubstance fluids and for trace substances, equivalent equations hold.
Medium.singleState is a medium property and defines whether the medium is only described by one state (+ the mass fractions in case of a multisubstance fluid). In such a case one initial condition less must be provided. For example, incompressible media have Medium.singleState = true.
DynamicFreeInitial 
DynamicFreeInitial  Dynamic balance, Initial guess value 

FixedInitial 
FixedInitial  Dynamic balance, Initial value fixed 
SteadyStateInitial 
SteadyStateInitial  Dynamic balance, Steady state initial with guess value 
SteadyState 
SteadyState  Steady state balance, Initial guess value 
Modelica.Fluid.Examples Multiway connections of pipes and incompressible medium model 
Modelica.Fluid.Examples.DrumBoiler.BaseClasses Simple Evaporator with two states, see Astroem, Bell: Drumboiler dynamics, Automatica 36, 2000, pp.363378 

Modelica.Fluid.Examples.HeatExchanger.BaseClasses Simple heat exchanger model 

Modelica.Fluid.Examples.HeatExchanger.BaseClasses Pipe wall with capacitance, assuming 1D heat conduction and constant material properties 

Modelica.Fluid System properties and default values (ambient, flow direction, initialization) 

Modelica.Fluid.Pipes.BaseClasses Base class for distributed flow models 

Modelica.Fluid.Pipes.BaseClasses.FlowModels Base class for momentum balances in flow models 

Modelica.Fluid.Interfaces Lumped volume with mass and energy balance 

Modelica.Fluid.Interfaces Base class for a lumped momentum balance 

Modelica.Fluid.Interfaces Base class for distributed volume models 

Modelica.Fluid.Interfaces Base class for a distributed momentum balance 