8160eae2____GeoServer_2_7_x_User_Manual__Description
[Parameter] Envelope2D [Mandatory] Y [Description] 
Contains the envelope for this mosaic formatted as LLCx,LLXy URCx,URCy (notice the space between the coordinates of the Lower Left Corner and the coordinates of the Upper Right Corner). An example is Envelope2D=432500.25,81999.75 439250.25,84999.75
Using the ImageMosaic plugin — GeoServer 2.7.x User Manual