The requirements for various surgical knots depend on the use. For example, a typical operation may require one to tie one hundred or more knots. None can fail. In these cases, the important elements are consistency, speed and efficiency, ability to securely appose tissue or ligate blood vessels, and simplicity. In some types of surgery, like arthroscopic procedures, one knot may have critical importance and the most important features may be low profile, ability to resist cyclical loading, perfect apposition without being either too tight or too loose, and ability to be formed by tying through a tubular structure.
There are solid evidence based reasons to use a surgeon's knot over a reef/square knot for skin closures with monofilament suture. The reef knot will not adequately appose the tissue as it loosens during tying. Pulling up on both ends of a suture tears small blood vessels and can saw through tissue, so that is not recommended. For most emergency room suturing, a Surgeon's knot - or the modification on the first throw where both ends are pulled in the same direction to lock it - works fine. It is easy to standardize with hand or instrument tying.
You would know better than I the proper definition of a noose, but basically they are knots where one side acts as a post. To distribute tension on a wound closure, suturing is often done with a running suture, and the knots at the ends are more resistant to breakage if they are tied as a noose rather than flat. I'm not sure what you are asking about defining "flat" but it is a common expression in surgery. Knots tied flat have proper knot conformation (like a typical reef knot) and suture tension is equal at each end. They also are properly dressed, with the knot lying at the level of the tissue and without looseness in the conformation.
