I just put General Recommendation, but I do not think this matters all that much. I think it is so you can keep track of your letters in your Interfolio account and know who wrote you letters for what (in case you use Interfolio for other jobs/opportunities; or if you use the same person as a letter writer to write you a dental school letter AND a letter for some scholarship - you are able to clarify which is which). I think the main thing that matters is that you select that it is a Confidential letter and requires a signature. That's my best guess!