Here is what I found from a professional association--their definition of sponsor's reference letter is:
The sponsor's reference letter should be on company letterhead and include:
Date
Name of applicant
Type of work performed and professional ability
Character and ethical standing
Applicant's special interest in asthma, allergy and immunology
Length of employment
Professional Ability
General statement that you recommend applicant for membership
Relationship with applicant (friend, colleague, co-worker, associate, etc.)
