After you have created a chain, you can change it by adding or removing bones. For information on adding or removing bones after an envelope has been set, see Adding and Removing Deformers.
After a chain is created, you can still add bones to it. The bones are added to the end of the chain, just before the effector.
While you can't delete individual bones from a chain, you can branch-select a chain and then delete it. If there are elements that are children in that chain, and you want to keep them, make sure to cut their links before deleting (select the elements and click the Cut button in the Constrain panel), and then reparent them to the chain. For more information on parenting, see Creating Hierarchies [Scene Elements].
Another way to make it appear as if a bone is removed is to reduce its length to zero, as described in Resizing (Scaling) Chains.
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