Primary evidence is data collected at the event, usually an official record. Secondary evidence is data collected sometime after the event, which is not as reliable as a primary evidence.
The question is what sometime after the event mean... does it mean one minute, one hour, one day, one week, one month, one year, or one decade. I don't know the answer to this. The idea of the Confidence Level is to give an estimate of the reliability of the data.
There is always a blend between one condifence level to another level. If you select Primary evidence, it means the source is pretty reliable. A Primary evidence may be classified as Secondary evidence by someone else, however it is still reliable. If on the other hand, you classify a source as Questionable evidence, then it means there is no way this source could be a Primary evidence.
From http://www.progenealogists.com/genealogysleuth.htm
Primary source material is information that is gleaned "straight from the horse's mouth" (so to speak). It is abstracted from a vital record, court record, deed or other original record and contains the first recording of an event. These records are usually created very near to the actual time of the event by someone who was actually present or who was directly involved in the event. An autobiography is also a type of historical primary source document.
Secondary source material can often be viewed a bit like "gossip." Secondary source material is often recorded long after the event has occurred and almost always by someone not really associated with the event. Most of the GEDCOM type, undocumented family groups and ancestries placed on CD-Rom format by software companies are types secondary source information.