When you make a designation, you make a formal choice about something and tell the court in writing about your choice. For example, when you designate your expert witnesses, you tell the court what experts are going to testify for you in your trial. If you designate an expert, you will be allowed to have that person testify at trial, but you don't have to call the expert to testify if you change your mind. On the other hand, if you don't designate an expert, and you allow the deadline to pass, you will not be allowed to use that expert as a witness for you in the trial.