I'm not sure what is so unclear here, but an option for setting Defraggler as the default defragger for Windows is not a life-changing, apocalyptic-computer-destroying matter in which members such as YoKenny of davey are making it out to be. It's well within reason for suggesting this option to be within the Defraggler program as other programs such as UltimateDefrag offer this exact feature (which can be ticked on/off).
Not sure what davey is talking about..."What is being discussed here is suggestions for Defraggler development. We are not discussing changes to Windows." It is a suggestion for Defraggler, but I can understand if you're confused since you maybe young or if english is not your first language. You might wanna look up the word "option" for more understanding...I'd have to recommend the Oxford English dictionary tho' the ones that are free online should suffice.