When making jellied fruit products, pectin is a key ingredient so the product will gel. Some fruits do not need added pectin, but some do. Recipes are made using dry or liquid pectin. These types of pectin are not interchangeable.
Pectins are a group of pectic polysaccharides, or long-chain carbohydrate molecules. They contain chains of esterified galcturonic chains that have different binding sites in a liquid form versus the dry form of pectin. Therefore, how the gelling process works when using liquid versus dry pectin is very different. Liquid pectin is not rehydrated dry pectin. The acid content of liquid and dry pectin is also different which changes gelling properties.
Again, one cannot be substituted for the other. Use the pectin the recipe requires for best results.
Source: Dr. Elizabeth Andress, Univ. of Georgia Extension