Pectin is a unique gelling agent and is perfect for fruit since it uses acid to set, where most other gelling agentsused (gelatin, starch, agar,...) do not tolerate acidity well, and it has a very clean flavor profile. A high methoxyl (or HM) pectin is common for jellies, and is available in a range of DE (degree of esterification). The set is affected by solids level, but once you add the acid source, your time to deposit is limited before it begins to pre-gel. Acidifying to a pH of 3.1 will bring on a very rapid set, and a pH of 3.5 will be longer to set. When purchasing pectin ask if it has already been buffered. This helps slow the set. If not you can add your own, usually the buffer salt of the acid you use. So for citric acid, you add sodium citrate.
Many feel a citrus pectin is cleaner than apple. The level of pectin you use will vary depending on the type and level of fruit you use, as some already contain protopectins.