FileInfo.com - The File Extensions Resource

.PLG File Extension

File Type	Sibelius Plug-in
File Format	File
Common?	No
File Description	Plug-in that adds extra features to Sibelius music notation software; may include additional ways to manipulate MIDI data, such as extra music transposition functions.
Program(s) that open plg files	Mac / Windows: Avid Sibelius

Information from FileInfo.com - The File Extensions Resource.
http://www.fileinfo.com/extension/plg