This Tcl command is used for working with versioned libraries that provide functionality to Tcl and Tk. It is structured into subcommands.
[http://www.tcl.tk/man/tcl/TclCmd/package.htm%|%man page%|%]
[Is there no succinct reference that explains the crucial role of pkgIndex.tcl?!? CL will return to this, if no one else does.]
DGP --- The man page for pkg_mkIndex, http://www.purl.org/tcl/home/man/tcl8.5/TclCmd/pkgMkIndex.htm covers the basics in the HOW IT WORKS section.
[Good point.] It's crucial that package authors understand pkgIndex.tcl as a keyword in package preparation.
DGP --- For now, yes, but stay tuned for better ideas. We don't want to be chained to pkgIndex.tcl files forever.
See also pkg_mkIndex pitfalls.
[One summary statement: package use currently requires comprehension of pkgIndex.tcl (see Magic names).]