Often you discover conflicting facts for an event, like a birth date. A particular source may give only partial information, like the place of birth but not the date, or the month and year but not the day or place. Soon you have a list of multiple alternate facts, each cited by a different source.
A “preferred” fact is a best estimate. It may include information from a combination of alternate facts. A single preferred fact is often used in genealogical reports and charts, where listing several alternate facts is infeasible.
For some facts, it is not a matter of having several conflicting facts from alternate sources. Sometimes you must draw inferences from several sources to form a conclusion. Suppose you have a hunch that a particular person is your ancestor. If you evaluate several sources, you may find proof to a reasonable degree of certainty that your hunch is correct. In other words you may be able draw a conclusion even though there are no explicit facts that prove it.
As an example, marriage records often list witnesses, their ages, and their relationship to the bride or groom. Evaluating the marriage records of two sisters may lead you to conclude that one of the witnesses who appears on both records is actually their brother. The age and hometown of this witness may lead you to conclude he is indeed your ancestor.
Therefore, besides citing individual sources for a given fact, you can also cite a conclusion. Simply document the steps that led to your conclusion, and name that as your source. Future researchers can then see your logic and verify it against your sources, and any new sources that may be discovered.