The string command accepts a special syntax for indices, which includes addition (but not the full power of expr) [1]. Maybe you are referring to this? Also, list commands accept the same syntax for list indices.

Example:

    % string index abc 1
    b
    % string index abc 1+1
    c
    % string index abc 1+1+1
    bad index "1+1+1": must be integer?[+-]integer? or end?[+-]integer?
    % string index abc end-1
    b

[1] http://www.tcl.tk/man/tcl8.6/TclCmd/string.htm#M54