Afaik the threshold indicates the degree of confirmation. 0 means that the tx isn't referenced by any tip, 100 means that all tips reference it. Anyone with more knowledge please correct me if I'm wrong.
In an envirnomnent with the coordinator running, the threshold can only be 0 or 100. Unconfirmed vs. confirmed. getBalances will only work with the value "100".
When the coordinator is shut down, the threshold can also be anywhere from 0 -100.