/*
 * ENUM: SE_UNION_TYPE_ENUM
 *
 *   Indicates the reason why <Features> have been grouped to create a
 *   <Union of Features>.
 */

Enumerator Notes

SE_SHARED_ATTRIBUTE

 The <Features> in the union all share one or more attribute
 values, and have been grouped so that the attribute(s) can
 be attached to the union rather than to each individual feature.

SE_COMPOUND_FEATURE

 The union represents a compound <Feature>, i.e., a <Feature> that
 is made up of multiple <Feature Topology> objects, e.g. a <Linear
 Feature> that is made up of multiple <Feature Edges>.

 For example, SEDRIS requires that all of the <Feature Edges> that
 make up a single <Linear Feature> be connected, but VPF doesn't,
 so sometimes a VPF compound line feature must be broken up in
 order to be represented in SEDRIS.  When this happens, all of the
 resulting <Linear Features> are placed in a <Union of Features>
 marked with SE_COMPOUND_FEATURE.

SE_COMPLEX_FEATURE

 The union represents a complex feature, i.e., a <Feature> that is
 made up of multiple <Features>.  The classic example is an airfield
 that is made up of runways, taxiways, control tower, hangars, etc.

SE_TILE_REFERENCE_FEATURE

 An <Areal Feature> that defines the spatial extent of a tile.

 In SEDRIS terms, a tile is a branch of a <Spatial Index Related>
 or <Perimeter Related> aggregation, such as a <Spatial Index
 Related Feature Topology> or a <Perimeter Related Features>.)

 Tile reference features can also show up as ordinary features;
 when they do, they are collected in a union with this label.

SE_UNKNOWN_UNION_TYPE

 If there was a reason for creating this union, we either don't know
 what it was or aren't telling.