A quick definition of informal application:

An informal application is a type of request for a patent that is not in the correct form as required by the U.S. Patent and Trademark Office. This means that it may be printed on both sides of the paper, or not be permanent, legible, or reproducible. However, it can still be corrected and retain the original filing date. Other types of patent applications include provisional applications, which establish a date for prior art and constructive reduction to practice, and divisional applications, which are based on the same disclosure as the original application but claim a different invention.