There seems to be lots of different opinions on what constitutes "shmura matzah," although most websites (and local Rabbeim) treat the most stringent opinion as the only opinion. There is a website that sells "soft matzah" that has the following to say regarding shmurah:
What is the Halachah?
The RIF rules that the Shemirah must be done from the time that the grain is cut.
The SHE'ILTOS (Parshas Bo) writes that from the moment the wheat comes in contact with water, it must be watched from becoming Chametz. The ROSH (2:26) rules that it is adequate to have Shemirah from the time of kneading. He adds that where he lives the mills are water driven and water is known to splash into the area where the flour is ground. Therefore it is appropriate to watch the wheat from the time that it is ground into flour. Furthermore,even the Rif who suggest that the flour should be watched before kneading, means only that it is preferable but not that it is critical.
The RAN argues that one must watch from before kneading which can only mean from a time when there is a risk of it becoming Chamets, grinding.
This appears to be the opinion of RASHI in Chulin (7a) as well, who mentions that we watch the flour from the time of grinding.
Some of these opinions are sourced, such as Rashi in Chulin. But where is the opinion from the Rif from? Or the Rosh. Are there any other big wig opinions on the matter?