The Name and Description elements are common to most SLD “objects”. The Name
identifies the well-known name of the layer being referenced, and is required. All
possible well-known names are usually identified in the capabilities document for a
server. The Description is informative.
The Layer Feature Constraints element is optional in a Named Layer and allows the
user to specify constraints on what features of what feature types are to be selected by the
named-layer reference. It is essentially a filter that allows the selection of fewer features
than are present in the named layer. This element is discussed in Sub clause 11.3 where it
is more apropos.
A named styled layer can include any number of named styles and user-defined styles,
including zero, mixed in any order. If zero styles are specified, then the default styling for
the specified named layer is to be used.
A named style, similar to a named layer, is referenced by a well-known name. A
particular named style only has meaning when used in conjunction with a particular
named layer. All available styles for each available layer are normally named in a
capabilities document.
The WMS interface uses the STYLES CGI parameter to reference named styles relative to
named LAYERS, as in the following example parameters:
LAYERS=Rivers,Roads,Houses&
STYLES=Center Line,Center Line,Outline
Parallel lists of corresponding layer names and style names are used in the CGI interface
because the CGI interface is not powerful enough to properly nest the named-style
references within the named-layer references. However, the SLD mechanism is. The
equivalent named-style selection mechanism in SLD is defined by the following DTD
fragment: