A list of items can be enclosed in double quotes to make it one argument. It can be still interpreted as a list, but it represents just one value, i.e. the list itself:
set a "this is a list of items"
All substitutions are performed as usual, so the main purpose of this quoting type is to put together a list of words and interpret the special characters:
% set a "abc $argc [expr 1+1]" abc 0 2