An Add-On is basically a catch-all phrase used by everyone to simply describe purchasing additional points. People will say they purchased first in year xxxx and have added on three times since then. That really means that on three other occassions they purchased more points.
That said, Add-on's can take on several different meanings.
1. You purchase additional points at the same resort and for the same use year as your original purchase. This is probably the only true add-on. Those points are all grouped together and treated as one set of points, not two. Example, buy 200 points at OKW for June (like us), and later add-on another 30 points at OKW for June (like us). These points are now a 230 point package and treated as if we purchased 230 points up front.
2. Purchase additional points at the same resort but a different use year (like us - through a resale). Example now 230 points at OKW, buy 100 point resale but August use year. These are two separate contracts and treated separately as a 230 point package and a separate 100 point package. When making ressies I must make sure MS knows which points to use.
3. Purchase additional points at a different resort. (like BCV's). Makes no difference whether they're the same use year as your initial purchase or not. These points are treated separately from your initial purchase. Again you must make certain MS knows which points you want to use when you make a reservation.
In general discussions all the above are reverred to as add-on's. But how the points are handled does vary.
Hope this helps.