A letter of assurance is used in a business setting to ensure the individual who receives the letter of something specific from the letter writer. An example of this would be a business selling something to another business, when the seller provides a letter of assurance for the item's condition.
Independent contractors or temporary employees may also receive a letter of assurance from a company. This letter could be used to assure the worker a specific number of hours or that a position is reinstated after a short hiatus. Any letter of assurance should be written on company letterhead and signed by all involved parties.