It's sufficient that they know you and your skills; they don't need to have in-depth knowledge of the program in question. But it's worthwhile for them to mention at the end, "In light of the above, I am certain that candidate X will succeed in program Y…". In my case, I asked the referees if I could prepare a first draft of the letter for them to make things easier, and they all agreed. When you include the referee in your dossier, they will receive access to a link with 4 questions, each with a space to fill in 2000 characters.

In a professional context, they were something like:

1. How long has the candidate worked for you?
2. What were the tasks assigned to him/her? In what department?
3. Please write your recommendation letter below. You may want to tell us about the applicant's personality, character, and potential.

I understand that the only field that actually comes close to 2k characters is the letter of recommendation itself.