Yes, basically
1) You can't add documents once the application is received by CIO since they have no way to match incoming documents to applications without a file number
2) However you CAN send in additional documents to your visa office once CIO have assessed the application and given you a file number and sent the application to the visa office
So if you end up getting another set of IELTS results and they are better than the results which were originally sent, it is worth sending these to the visa office once you get your file number from CIO, since this can strengthen your position especially since the visa office do another eligibility review and they can occasionally contradict CIO's eligibility decision.
However if you want to send anything to the visa office it is very important to do this BEFORE the make a decision. Once an application is approved or denied they will not consider any additional documentation.
But as RKC mentions you can appeal a NER at CIO stage if it is unfair and it should have been a PER based on the documents CIO received with the application. CanadaJimmy has kindly posted a page on my blog (linked below) which explains how he successfully turned around a NER into a PER.
Wayne.
