What do square brackets mean in JMP/JSL?

[#1]

I have, in a loop:

to_update = dt << get rows where (num(dt:Number) == ID);
write(to_update);

giving

...
[199, 200]
...
[209, 210]
...

What do square brackets mean, therefore?

[#2 · Accepted solution]

Square brackets refer to matrices.
a = [ 1, 2, 3 ];
or they refer to subscripts in lists
bList = { "A", "B:, "C" };
x =blist[1];

I suggest you take the time to read the Scripting Guide, which is found in the JMP Documentation Library, found under the Help pull down menu. It will give you the details of the structures and syntax of JSL.