Can a sponsorship letter be used for the medical soft power DTV application in Thailand?

Under the information about the DTV on the info page is says

"Financial evidence: amount of no less than ฿500,000 ( $ 14,750+ ) for the last 3 months, e.g. bank statements, payslips, sponsorship letter"

Does sponsorship letter work for all softpower?

If i do a medical soft power application what does this sponsorletter need to say?

My impression is that a sponsorletter is to secure and cover your stay, but should that then say 5 years or 180 days??

Anyone used a sponsorletter on a softpower dtv?