Accumulo Iterators SortedKeyValueIterator source
The whole iterator stack is constructed on the tablet server (TServer).
At the bottom of the stack, are the built-in system iterators that read your data, merged from memory/files. Just on top of that are system iterators that implement access control and pruning of delete markers. At the top, are all your user iterators, ordered by the priority you set in your per-table and/or scan configuration.
The init method is called by the TServer during the construction of this stack of iterators, and it handles passing the one iterator on to the init method of the next in the stack.