You may not particularly like a past employee and you may not wish to advise an employer to take them on, but you can still give them a useful reference. This is because some things needed for employment are routine and basic. They may not have given you what you expected but they may be good in their next job, which is more their cup of tea. This "General Reference" is a "to whom it may concern" form of reference that discusses experience in the past but does not discuss suitability in the future. It is "to whom it may concern" because the referee doesn't want to get involved. In this case the applicant has asked for a reference but there is no discussion about the new position or their suitability. While it reads like a letter, this example is like filling in an interrogative form about years employed, hours worked, honesty, reliability, etc.
"General Reference" OR "to whom it may concern" that discusses experience in the past but does not discuss suitability in the future.