curved brackets define discreet parts of a statement.
for example 1 + 2 * 3 = 7, because the multiplier has precedence. (1 + 2) * 3 = 9, because the brackets keep the first bit discreet.
square brackets are used in conjunction with arrays, for example loon setpos [getpos loon select 0, getpos loon select 1+2]
curly brackets are interchangeable with "" quotes, so the above example could just as well be written
("_x in thislist" count units group_one) + ("_x in thislist" count units group_two) >= 4
thus indeed, the question is answered. and if anyone needs a test missionette to prove it, just ask.