The DER must have thorough knowledge of his or her
responsibilities under the Federal Regulations (49 CFR Part 40). The DER
has authority to make decisions about the testing process and answer
questions about it.  In order to make proper decisions and handle all
aspects of the DOT drug & alcohol testing program, the DER needs a complete
understanding of the Rules and his or her duties under the Federal
Regulations.
Every company covered by Department of Transportation Regulations that employs safety-sensitive employees, must have one (or more) Designated
Employer Representatives (DERs).

Click the link below for more information:
https://certifiedtrainingsolutions.com/designated-employer-representative.html