In the URL or catalog::Modifier command sequence, a layer definition sequence starts with the layer= command and terminates with another layer= command, an effect= command, or the end of the URL.
All commands within the layer definition sequence are associated with the layer.
layer= command specifies a layer number, which must be an integer 0 or larger. All commands in layer definition sequences with the same layer number are applied to the same layer. If the same command occurs more than once, the last instance will prevail.