Problem with a list in CAS-program

[webmasterpdx]

I've asked this question before with (), {}, [] etc, I've only seen () used to index matrices or lists, so where is [] being used to index a list coming from?????
There needs to be consistency in the indexing of variables of a certain type. What are we supposed to do, guess as to which brackets to use?
...i.e. Where in the documentation does it refer to using [] to index a list?
Thx