http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/tclvars.htm 

[[Someone want to comment on where this global variable
gets set and to what good uses one can put it to?]]

[RS]: Contains the error message and [traceback] of the last error. Silly example:
 % proc foo {} {bar x}
 % proc bar {input} {grill$input}
 % foo
 invalid command name "grillx"
 % set errorInfo
 invalid command name "grillx"
    while executing
 "grill$input"
    (procedure "bar" line 1)
    invoked from within
 "bar x"
    (procedure "foo" line 1)
    invoked from within
 "foo"


See also [errorCode].
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[Tcl syntax help]