>> when we design flow type, how many flow type we needed for one transaction type
[A.]: there are no strict rules how many Flow types should be assigned to one Transaction type - it depends both on context and instrument itself. E.g. in case of FX, you will need as a minimum 'Buy foreign exchange' and 'Sell foreign exchange'. Though if you are dealing with non-convertible currencies, then you also need 'Cash settlement'.
>> there are a lot of documents posted, if we only use one flow type, can we combine all accounting doc into one document, why we need so many account document?
[A.]: what is the issue of having more than one accounting document per one Financial instrument?
>> the last one dones not generated any accounting doc, it says: fixed not for posting, I don't know why we need this?
[A.]: I assume you mean those Update types, which are Fixed, but not posted - the reason - they are not relevant for posting. Those could be required for calculating position or other internal purposes - e.g. in case of Interest rate Swap (IRS) you specify nominal amounts, though these are purely notional amounts - they are not relevant for posting.
P.S.: it is a good practice to address one question per Post. This also means you avoid addressing new questions in the same Post.
Kind regards,
Renatas