Let’s imagine you are reading a chart, looking for a nice place to “drop a hook”: (LANDLUBBER translation: anchor). You see a nice cove on the chart, and then you see some odd letters in that cove. They might read: S (or maybe) M (or perhaps) Si . And that would make you happy.
On the other hand, you might see some of these letters: Rk (or maybe) Co (or perhaps) Wd! . And that might make you sad. Sad enough to seek out another anchorage!
So what in the name of alphabet soup are you reading on the chart? Answer: the condition of the bottom, or seabed. Why should you care what is on the bottom? You probably shouldn’t care – unless you plan to anchor.
Thus: S = Sand; M = Mud; Si = Silt. Pretty good for anchoring as you can sink an anchor into these bottoms and the holding power is great.
However: Rk = rocky; Co = Coral [huge fines possible if you try to anchor in coral]; and Wd = Weeds. None of them making a friendly place for your anchor to visit.
Your chart tells you this, and might help you decide if a Bruce anchor, or a plow, or a Danforth anchor might work better – or the chart might convince you not to anchor at all!
So what’s the “bottom line” to the letters on your chart? Answer: Giving you enough information for securely attaching your anchor TO the bottom!