The command symbolevery n instructs plotl to only put symbols on every nth point. The default is to put a symbol on each point (that is, symbolevery 1).
This command is useful when the nuber of points plotted is large enough that putting a symbol on each one would clutter the figure.
The command plotevery n instructs plotl to only plot every nth point. The default is to plot every point (that is, plotevery 1).