A letter of reference is a relatively simple documentation of acting as a reference to a person, defining the basis of the working relationship, and expressing an opinion of the person's skills and abilities. Please note that by definition it is not a letter of recommendation.
Examples of Letter of Reference:
'I was Mr. ABC's manager in his role as senior potato adviser for 5 years at XYZ Spuds Inc. Mr. ABC was an excellent, highly efficient employee with strong professional skills and excellent talents in working with our clientele. I found Mr. ABC's work with us to be of the highest standard. I will be happy to act as a referee on his behalf.
Yours sincerelyMick Murphy
XYZ Spuds Inc.'