This Tcl command is used for working with versioned libraries that provide functionality to Tcl and Tk. It is structured into subcommands.
DGP, PYK: The man page for pkg_mkIndex covers the basics in the HOW IT WORKS section. For now pkgIndex.tcl is the primary way to make Tcl aware of packages, but stay tuned for better ideas. We don't want to be chained to pkgIndex.tcl files forever.
See also pkg_mkIndex pitfalls.