address comments

include/aspect/simulator.h

@@ -1194,7 +1194,7 @@ namespace aspect
                                         LinearAlgebra::BlockVector &tmp_distributed_stokes);
 
       /**
-       * Offset the boundary id of all faces located on an outflow bounday
+       * Offset the boundary id of all faces located on an outflow boundary
        * by a fixed value given by the input parameter
        * @param boundary_id_offset.
        *
@@ -1210,7 +1210,7 @@ namespace aspect
        * This function is implemented in
        * <code>source/simulator/helper_functions.cc</code>.
        */
-      void reset_outflow_boundary_ids(const unsigned int boundary_id_offset);
+      void restore_outflow_boundary_ids(const unsigned int boundary_id_offset);
 
       /**
        * Remove the linear momentum of the given vector

source/boundary_composition/interface.cc

@@ -331,11 +331,22 @@ namespace aspect
                            "When the composition is fixed on a given boundary as determined "
                            "by the list of 'Fixed composition boundary indicators', there "
                            "might be parts of the boundary where material flows out and "
-                           "one may want to prescribe the composition only on the parts of "
+                           "one may want to prescribe the composition only on those parts of "
                            "the boundary where there is inflow. This parameter determines "
                            "if compositions are only prescribed at these inflow parts of the "
                            "boundary (if false) or everywhere on a given boundary, independent "
-                           "of the flow direction (if true).");
+                           "of the flow direction (if true)."
+                           "Mathematically speaking, the compositional fields satisfy an "
+                           "advection equation that has no diffusion. For this equation, one "
+                           "can only impose Dirichlet boundary conditions (i.e., prescribe a "
+                           "fixed compositional field value at the boundary) at those boundaries "
+                           "where material flows in. This would correspond to the ``false'' "
+                           "setting of this parameter, which is correspondingly the default. "
+                           "On the other hand, on a finite dimensional discretization such as "
+                           "the one one obtains from the finite element method, it is possible "
+                           "to also prescribe values on outflow boundaries, even though this may "
+                           "make no physical sense. This would then correspond to the ``true'' "
+                           "setting of this parameter.");
       }
       prm.leave_subsection ();
 

source/boundary_temperature/interface.cc

@@ -362,7 +362,19 @@ namespace aspect
                            "the boundary where there is inflow. This parameter determines "
                            "if temperatures are only prescribed at these inflow parts of the "
                            "boundary (if false) or everywhere on a given boundary, independent "
-                           "of the flow direction (if true).");
+                           "of the flow direction (if true)."
+                           "Mathematically speaking, the temperature satisfies an "
+                           "advection-diffusion equation. For this type of equation, one can "
+                           "prescribe the temperature even on outflow boundaries as long as the "
+                           "diffusion coefficient is nonzero. This would correspond to the "
+                           "``true'' setting of this parameter, which is correspondingly the "
+                           "default. In practice, however, this would only make physical sense "
+                           "if the diffusion coefficient is actually quite large to prevent "
+                           "the creation of a boundary layer. "
+                           "In addition, if there is no diffusion, one can only impose "
+                           "Dirichlet boundary conditions (i.e., prescribe a fixed temperature "
+                           "value at the boundary) at those boundaries where material flows in. "
+                           "This would correspond to the ``false'' setting of this parameter.");
       }
       prm.leave_subsection ();
 

source/simulator/core.cc

@@ -744,7 +744,7 @@ namespace aspect
       }
 
     if (!boundary_temperature_manager.allows_fixed_temperature_on_outflow_boundaries())
-      reset_outflow_boundary_ids(boundary_id_offset);
+      restore_outflow_boundary_ids(boundary_id_offset);
 
     // If there are fixed boundary compositions,
     // update the composition boundary condition.
@@ -784,7 +784,7 @@ namespace aspect
       }
 
     if (!boundary_composition_manager.allows_fixed_composition_on_outflow_boundaries())
-      reset_outflow_boundary_ids(boundary_id_offset);
+      restore_outflow_boundary_ids(boundary_id_offset);
 
     // let plugins add more constraints if they so choose, then close the
     // constraints object

source/simulator/helper_functions.cc

@@ -1872,7 +1872,7 @@ namespace aspect
 
   template <int dim>
   void
-  Simulator<dim>::reset_outflow_boundary_ids(const unsigned int offset)
+  Simulator<dim>::restore_outflow_boundary_ids(const unsigned int offset)
   {
     // Loop over all of the boundary faces...
     typename DoFHandler<dim>::active_cell_iterator