What is the difference between liquid and powdered pectin? We have powdered pectin and have a recipe that calls for liquid. What are the differences between the two? How are they made? How do their uses differ? How do you substitute one for the other? what changes to a recipe will it require?

This seems like it should be fairly straight forward, but the Internet is being no help.
I found the following on the internet

Use only the type of pectin called for in your recipe. Powdered pectin and liquid pectin are not interchangeable in recipes. The preserving books seem to confirm that the reason liquid and powdered pectin may not be interchangeable is that the liquid version is always added after boiling but most types of powdered are added to the raw fruit or juice. 

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