Tcl attempts to match the name of any entered command as an abbreviation of a valid command (either a Tcl or XSPEC command). If the entered command matches more than one valid command, Tcl then lists the possbile choices, but does not execute the command. In the case of XSPEC commands, aliases have been constructed matching the command to its minimum abbreviation, as listed when typing ? at the XSPEC prompt. For example, the minimum abbreviation for the plot command is p. Thus, typing p will execute the plot command, even though this would otherwise be ambiguous with the Tcl commands pid, proc, and puts.
Command completion is implemented using the Tcl unknown procedure, which is a script file loaded by Tcl at run time and may be different or not exist on your system. See the section below on the unknown command for more details on how it is implemented in XSPEC.