These are the bugs that developers can not solve it due to some constraint or limitations.
Whenever the developer accepts the bug but he is not able to fix the bug as that bug fix may lead to affect whole application is called Can't fixed status.
Following are the reasons for Can't fixed/Won't fixed bugs:
Bug is in the core of application.
No technology support.
Cost of fixing bug is more than retaining it.
Deferred means postpone in which developer postpone the bug and fixes it in the next release.
Deferred can't be done in the initial stage of a build, it can be done in the last stages of the build.
Following are the reasons for Deferred/Postpone bugs:
Due to time constraint.
Due to impact area.
Request for Enhancement
These are the suggestions given by the test engineers towards the enhancement of the software or the application.