Working to a brief.

A brief is a set of signed rules/guidelines set by the client for the worker to follow, if the rules of the brief are broken then the worker will not have delivered the product that was promised and the agreement will have been broken. If a client makes a deal with workers then those workers have been commissioned, this basically means that they have been hired to create whatever it is the client has asked for, they will be responsible for script writing, locations, casting, etc. so they are basically tasked with the pre-production process. In terms of media tender just means to wary of your target audience to ensure that you don't offend anybody. You could also respond to a brief by entering a competition rather than being commissioned.
 When entering the Esting competition we were not allowed dialog, real violence, gore, etc. so we had to be mindful of that, we were also limited to keeping it ten seconds long, we had to work around the guidelines just to be allowed into the competition. We had no room to negotiate the brief as we were not commissioned to make it and we submitted it voluntarily. As mentioned earlier we were asked  to keep the entries family friendly, however, we had complete freedom over the genre, style and narrative of our animation. I did not have to change my final idea whatsoever because I met the guidelines completely. A client will usually set a deadline and a budget because they plan to release the film as quickly and as cheap as possible without sacrificing the quality of the final product.
Someone may respond to a brief because they think that it may work better in a certain way, whether that improves the quality, saves money, etc. While working on the Esting brief I learned how to create lightning effects in after effects which may prove useful in the future if I decide to pursue a career in media. While making the animation I took on all of the roles, director, camera work, editing and sound, this made decision making a lot easier and sped up the whole process because I did not have to consult anyone, however, it did mean that it was harder to get another opinion.
There are a few different types of briefs:

"According to Wright, the schism was born out of Marvel’s desire to create new draft of the movie without Wright and Cornish. “Suddenly becoming a director for hire on it, you’re sort of less 

emotionally invested and you start to wonder why you’re there, really.”"- Nerdist June 2017.

This is the winner of a competition of the website named BlueCat who has gained profit and notoriety,etc.
Cadaver Dog by Judy Soo Hoo & Isaac Ho
When a young autistic Latina rescues a stray cadaver dog, she stumbles upon a 15-year-old cold case that no one wants solved.
Read more at http://www.bluecatscreenplay.com/past-winners-finalists/#sMcHm1ueZZaVpx80.99

 CONTRACTUAL: A legal brief written by employer and employee which secures rules and guidelines of the job that the employer is contracting the employee for.
FORMAL: A document that holds detailed information about what the employer wants the company to achieve without wasteful information.
INFORMAL: A verbal outline of what and how the employee should do their job and the employer and employee will agree the details of the contract.
CO-OPERATIVE: Two or more companies are hired to work on the same project\job where they will work as a single team. All companies involved will be held responsible if the deadline is missed or the client is not happy with the outcome.
NEGOTIATED: This would be brief where the client and the employee will both discuss and agree on the job and how it will be completed as well as  the payment, deadline, etc.
COMISSION: A large company will employ a smaller company to basically do their work for them but all rights to the project will go to the client.

This is a screenshot from the E4 website showing the terms and conditions of the competition and proving the legislation of the competition.

SOURCE: https://johnrichharrison.wordpress.com/2014/10/24/e4-sting-brief/
https://www.wikipedia.org
http://coolerinsights.com/2012/08/how-to-write-a-social-media-brief/
http://estings.e4.com/terms.html
https://nerdist.com/edgar-wright-reveals-why-he-left-ant-man/
http://www.bluecatscreenplay.com/past-winners-finalists/