visa category and docs needed

I am a Sri Lankan and have been invited to China by a medical school to evaluate their ethics review committee for accreditation. I have a letter sent by them via email.
1. What is the most appropriate visa category for me? Looking at the web site details it appears to be F.
2. For category F the following documents are needed
1、Original “Invitation Letter of Duly Authorized Unit”（被授权单位邀请函）or “Confirmation Letter of
Invitation”from Duly Authorized Unit (被授权单位邀请确认函原件)
2、An invitation letter issued by a relevant entity or individual in China.

What is the difference between 1 & 2? The Chinese med school officials are not clear on this and want a clarification. Appreciate a quick response as I need to be in China by 6th September the latest and need to apply for visa asap. I received the invitation only on 23rd August.
3. How long would it take to get the visa once documents are handed over?
Thanks
Dr C W Sri Lanka