The vertical bar, pipe, v-bar, etc. is used for many things. In AutoCAD it designates an element, object, or entity that originates in an externally referenced file. Most savvy AutoCAD users know that if they encounter a layer name with this symbol then the layer is actually found on the filename listed in front of the pipe symbol. The usage also occurs in the "Image manager" where it designates a raster image file that is brought into the current drawing through the external reference it was originally inserted in. Can't detach that image? Open the xref and detach it there!
Other uses for the vertical bar? Check out wikipedia