Tcl allows you to abbreviate any command to the closest non-ambiguous substring. For instance:
% string range abcd 0 2
abc
% stri ran abcd 0 2
abc
% str r abcd 0 2
unknown or ambiguous subcommand "r": must be bytelength, compare, equal, first, index, is, last, length, map, match, range, repeat, replace, reverse, tolower, totitle, toupper, trim, trimleft, trimright, wordend, or wordstart
%
The final line has an ambiguous match - r can match any of range, repeat, replace or reverse so we get an error raised. ra
is the shortest match possible for range
here.
The specific documentation applicable to this is actually in the man page for Tcl_GetIndexFromObj where is states
A match occurs if objPtr's string value is identical to one of the strings in tablePtr, or if it is a non-empty unique abbreviation for exactly one of the strings in tablePtr and the TCL_EXACT flag was not specified
This is because the Tcl_GetIndexFromObj
API call is typically how commands and subcommands are identified throughout the Tcl sources.