"Impeachment purposes" means evidence to challenge the validity of other presented evidence or testimony. In a civil case, you generally make a set of claims and provide evidence to back them up.
The discovery disclosures in the rule (and this rule shows up all over federal civil procedure) is that you are exempted form the providing the disclosure information for sources that you are going to use exclusively to challenge the other side's evidence/testimony validity, not to make any point of your own.
For example, if the other side is going to have an expert witness who is a doctor, my evidence that he had his medical license revoked is not required to be disclosed.