An "original" classifier is someone who deems a new type of information as a certain classification level. For instance, if we invented a new technology to address a new threat -- information associated with that technology and threat will need to be assessed for classification. An original classifier would be responsible for assessing, at a high-level, whether the release of that information would constitute damage to the government and, depending on severity of damage it would cause, would deem it unclassified, confidential, secret, or top secret. Original classification tends to be more categorical than specific.
Once an original classifier makes that determination for a type of information, a "derivative" classifier would use that information to officially classy a specific document. As such, derivative classification is much more common.
Unfortunately, my guess is Hillary was neither and was just depending on the information being classified (and marked accordingly as it came in). As an information generator, however, my opinion is that she should have reached out to a classifier to determine the proper classification if there was any chance at all the information was classified.
