By only plotting the Demand Below Supply Above indicator you get automatic SD level that is updated every time reaches either 30 or 70. If you plot the / you get an indicator that extends the zone instead of overwrite the earlier zone. Due to the zone being extended the chart can get a bit messy if there isn't a clear range going on.
There is also a "confirmation bars" setting where you can tell the script how many bars under over 30 / 70 you want before a zone is drawn.
Here is an image of only using the "Demand Below / Supply Above" plot.
As you can see, this could be useful "Price Flow" indicator, where we would only short if a zone appears below another zone, or long if two zones in a row are going up, like stairs.