From: Wolfgang Bangerth <bangerth@colostate.edu>
Date: Mon, 12 Aug 2019 13:20:13 +0000 (-0600)
Subject: Update some documentation pieces.
X-Git-Tag: v9.2.0-rc1~1258^2
X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=4f4ba6134784ed70798424b7441af906e78f3de2;p=dealii.git

Update some documentation pieces.
---

diff --git a/include/deal.II/grid/tria_accessor.h b/include/deal.II/grid/tria_accessor.h
index 3c53e712f2..f99b969970 100644
--- a/include/deal.II/grid/tria_accessor.h
+++ b/include/deal.II/grid/tria_accessor.h
@@ -3260,7 +3260,11 @@ public:
 
   /**
    * Get the level subdomain id of this cell. This is used for parallel
-   * multigrid.
+   * multigrid where not only the global mesh (consisting of the active cells)
+   * is partitioned among processors, but also the individual levels of the
+   * hierarchy of recursively refined cells that make up the mesh. In
+   * other words, the level subdomain id is a property that is also defined
+   * for non-active cells if a multigrid hierarchy is used.
    */
   types::subdomain_id
   level_subdomain_id() const;
diff --git a/include/deal.II/grid/tria_levels.h b/include/deal.II/grid/tria_levels.h
index 9ec92745f6..70c513ad36 100644
--- a/include/deal.II/grid/tria_levels.h
+++ b/include/deal.II/grid/tria_levels.h
@@ -120,13 +120,19 @@ namespace internal
       /**
        * One integer per cell to store which subdomain it belongs to. This
        * field is most often used in parallel computations, where it denotes
-       * which processor shall work on the cells with a given subdomain
+       * which processor shall work on/owns the cells with a given subdomain
        * number.
+       *
+       * This number is only used on active cells.
        */
       std::vector<types::subdomain_id> subdomain_ids;
 
       /**
-       * for parallel multigrid
+       * The subdomain id used on each level for parallel multigrid.
+       *
+       * In contrast to the subdomain_id, this number is also used on inactive
+       * cells once the mesh has been partitioned also on the lower levels of
+       * the multigrid hierarchy.
        */
       std::vector<types::subdomain_id> level_subdomain_ids;
 
diff --git a/source/grid/tria_accessor.cc b/source/grid/tria_accessor.cc
index 06321aebf5..80579e6e84 100644
--- a/source/grid/tria_accessor.cc
+++ b/source/grid/tria_accessor.cc
@@ -1981,6 +1981,7 @@ CellAccessor<dim, spacedim>::set_subdomain_id(
 }
 
 
+
 template <int dim, int spacedim>
 types::subdomain_id
 CellAccessor<dim, spacedim>::level_subdomain_id() const