A "written acknowledgement" means getting something in writing that shows that someone has received or recognized something. It can be a letter, email, or any written document that officially acknowledges or confirms something. Full definition
Not all new hires need to sign an Employment Agreement, but it's a good idea to have in writing some sort of written acknowledgement of terms and conditions. (rocketlawyer.com)
In writing acknowledgement letters, you need to be courteous and sincere. (officewriting.com)
Writing acknowledgements actually feels almost like its own genre; it has a very specific purpose and audience. (edutopia.org)