From the SLD Spec (1.0.0):
Defining a user-defined style requires information about the features being symbolized, or at least their feature type. Since user-defined styles can be applied to a named layer, there needs to be a mechanism by which a client can obtain feature/coverage-type information for a named layer. This is another example of bridging the gap between the WMS concepts of layers and styles and WFS/WCS concepts such as feature-type and coverage layer. To allow this, a WMS may optionally support the DescribeLayer request. This can be applied to multiple layers as shown in the example below:
http://yourfavoritesite.com/WMS?VERSION=1.1.0&REQUEST=DescribeLayer&LAYERS=Rivers,Roads,Houses
where DescribeLayer is a new option for the REQUEST parameter and LAYERS is the parameter that allows a number of named layers to be specified by name. This is thought to be a better approach than overloading the WMS Capabilities document even more.
The response should be an XML document describing the specified named layers. If any of the named layers are not present, the response is an XML document reporting an exception.
For each named layer, the description should indicate if it is indeed based on feature data and if so it should indicate the WFS/WCS (by a URL prefix) and the feature types. Note that it is perfectly valid for a named layer not to be describable in this way. It has been suggested that we reuse the WFS mechanism for indicating how one identifies feature types in a WFS, namely by using the Query element. Annex B gives the DTD for the response.
<?xmlversion="1.0"encoding="UTF-8"?><Unable to render embedded object: File (//some.site.com/sld/DSR.dtd"><WMS_DescribeLayerResponseversion="1.1.0"><) not found.-'Layer_A'comesfromthewfsspecifiedbytheprefix"http://www.mywfs.com/WFS?"andhasfeaturesoftypes'Road_FT'and'Route_FT'><LayerDescriptionname="Layer_A"wfs="http://www.mywfs.com/WFS?"><QuerytypeName="Road_FT"/><QuerytypeName="Route_FT"/></LayerDescription><!'Layer_B'cannotbedescribedintermsofaWFSandsohasnowfsattributeandnocontents-><LayerDescriptionname="Layer_B"></LayerDescription></WMS_DescribeLayerResponse>
Note that even an integrated WMS should provide a WFS/WCS prefix, since this allows
DescribeFeatureType requests to be made.
A WMS need not support the DescribeLayer request. However, it should be noted that it may be common for a WMS not to support UserLayers but to allow UserStyles to be applied to named layers. In such circumstances, supporting the DescribeLayer request is the only interoperable way in which a client can specify user-defined symbolization.