Each rundown is designed to handle a specific area of a person's case on the Ls. These levels were made originally by L. Ron Hubbard for executives to boost their power without doing the entirety of auditing actions available.
The Ls are not grades in themselves but incredible boosters to beingness, doingness, and havingness in life. Primarily, the tech comes from research into increasing OT powers.
They're a particular class of rundown - they increase your ability and effectiveness in life by magnitudes through the use of an auditing tech called listing and nulling - one of the most effective and powerful auditing techniques ever developed.