The InstallRig plugin is explained in the video tutorial I put together a few years ago...here (it explains how to build a rig that uses the plugin and what it will do).
The video referenced does address the plug-in, but it also addresses a simple "squetching scheme" first, and then eventually demonstrates creating a complicated INSTALL rig. For my own purposes, after watching, I decided to document what I gleaned from the video, because I ...well...uh...forget...if I don't write it down.
So in case this summary might be useful for others, I offer it here. Please David, Mark, correct any mistakes in my description, or add info:
One uses the INSTALL RIG plugin usually when when one wants to install an already existing (usually complicated) generic rig, in multiple models. The generic rig has specially named bones that conform to the syntax assumed by the INSTALL Rig plug-in. So far, I am aware of only the 2008, Squetch generic rigs that use this convention. One doesn't normally use the plug-in when creating a rig from scratch.
The existing generic rigs (aka install rigs) will have geometry bones named with convention: bonename_geom & name_INSTALL bones (that act like commands to the plug-in) with syntax:
[+ ] anyname_INSTALL [bone_in_model [ /another_bone_in_model [ (attached_to_parent)][/...]] ]
+ more bones
where  =optional
The bone_in_model name is the same name as some bone in the new model or the install rig model, eg Hips, Head, etc. If there is no bone_in_model named, the bones will be inserted in place (only the INSTALL bone will be deleted). One can also optionally indicate if the bone, when re-parented should be attached to it's parent.
How to use plug-in:
1) Import the generic INSTALL rig into specific model (generic rig may or may not include poses). The new bones come in unparented, at top level of hierarchy. Usually parts are isolated, so as to make the next step easier to:
2) Position, orient the geometry & control bones of the generic rig bones to line up with the geometry of the specific model, usually by positioning, orienting the INSTALL bones (but not limited to)
3) Then in bones mode: right click in window or on model in PWS/plug-in/wizards/Install rig.
The Plug-in will parent the bones according to the INSTALL command bones, delete all the INSTALL bones, unhide any bones with the string Geom (not case sensitive) in their name and hide all other bones in Bones Mode.
If you have your own generic rig, you could create your own INSTALL rig or do it all by hand (with your own naming conventions):
1) import (or drag) your already existing rig into any other model, and 2) resize, reorient, reparent geometry bones, control bones and 3) hide/unhide by hand.
Edited by NancyGormezano, 06 July 2012 - 08:35 AM.