For pilots, "jumpseating" is a negotiated privilege (not 'a right') between pilot groups (usually a 'quid pro quo' kind of agreement). It is quite literally the PIC of the aircraft exercising PIC authority to board you as the final authority as to the operation of the aircraft (note, neither the gate agent or lead FA is involved in this process). After 9/11, it was determined that the least amount of access to the flight deck (which, on many cargo aircraft, the entire airplane is designated 'the flight deck' as far as security is concerned) should be maintained as a safety concern by the government. As such, your identity must be verified before you can occupy the flight deck through an approved method, and you shall not access the flight deck unless there are no seats available for you in the cabin. It is the PIC's responsibility to verify the identity of all jumpseaters. The ops/gate agent is required to assist the PIC in verification of identity for flight deck access only because the PIC does not have access to an approved method of identity verification in the flight deck. So, if you're 'jumpseating', you're accessing the aircraft under the authority of the PIC. You sit in the cabin unless there are no seats available to occupy there. If you're accessing the flight deck because there are no seats in the cabin for you, you need to have your identity verified through an approved method (and be put on the General Declaration as a crewmember for international flights). CASS is for "domestic, off-line, flightdeck access" only. CASS is not an approved method for verifying the identity of jumpseaters for flight deck access internationally - thus no 'offline' flight deck access internationally. Nor is CASS an approved method for verifying on-line pilots (Delta pilots on Delta, US Airways Express pilots on US Airways, etc., though this seems to be misunderstood by 80% of pilots and 95% of gate agents). Nor is CASS an approved method for verifying the identity of anyone requiring cabin access - you shouldn't be run through CASS if you're not needing flight deck access and more so than your grandmother should be run through CASS for cabin access - approved identity verification is not required for cabin access. Most passenger airlines also require you to pay the taxes and fees, but not all the costs associated with your travel if you're jumpseating (international landing fees are usually calculated on a per passenger basis, so, if you are jumpseating and sitting in the cabin, the airline must pay additional $, so they pass on the cost to you).

Non-rev'ing is travel that is not for revenue. You are a passenger (your status as a professional pilot or the ability to access the flight deck as a jumpseater is irrelevant), and are accessing the cabin of the aircraft only, just as a revenue passenger would. Your access to the aircraft is granted not by the PIC, but rather by the airline - though, the PIC still maintains the final authority to remove you from the aircraft, as always. The gate agent controls this access (unlike jumpseating, where the PIC grants you access, not the airline through the gate agent). Generally, non-rev'ing is more expensive than jumpseating for many pilots b/c the airline charges you for all of the costs associated with carrying you (meals, amortized mx, crew costs, etc.), not just the taxes and fees associated with international travel (domestic fees are not charged to jumpseaters).

So, why non-rev then? Non revenue passengers virtually always have a higher boarding priority than jumpseaters. If there's only 2 flightdeck jumpseats and 3 applicants for jumpseat access, the lowest priority jumpseater is often left behind - had they non-rev'd, they may have had a higher priority than the lowest priority non-rev, and thus got on the aircraft (due to the nature of DOH seniority, many passenger airline CA's have enough seniority to have some of the highest non-rev priority on their own airline - the difference between a first class seat in the cabin as a non-rev or a cramped seat in the flightdeck as a jumpseater on the same flight). Domestically, some airline's non-rev costs are paid in a one-time fee, so there's no financial loss to non-rev on your own airline.

It is a bit confusing that both non-rev'ing and jumpseating pilots often end up in the cabin together, but the difference is the authority that granted them access to the aircraft, the PIC or the airline (and, as 'robthree' points out, how much they paid out of pocket for the seat).

Hope this helps, rather than confuses the situation further.