A script containing package ifneeded command(s) that gets evaluated somewhere in the operations of a package unknown handler script.
In the default package unknown handler of Tcl releases 7.5 through 8.4, each index script is stored in a file with the name pkgIndex.tcl.
Tcl 8.5 added Tcl Modules, which construct index scripts on the fly, using little but file names.
Other package unknown handlers may have other means to either store and retrieve, or to construct on the fly, their index scripts.