Most, but not all, of the available layer controls are concerned with plotting table data in some way or other. In general, these allow you to set up a way to generate a plot layer (some generated graphics) from some selected columns of a given table with a chosen style.
However, if you have more than one Row Subset for a given table, a single control can be used to generate several layers, one for each subset. In most cases it makes sense to have the graphics generated by the different subsets configured similarly, but still visually distinguishable.
These table layer controls generally have three tabs: Position, Subsets and Form. The Position tab is for specifying coordinates, and depends on the specific type of the plot and the layer control. The Subsets and Form tabs control which row subsets are plotted and how they are displayed, and a description of how they are arranged follows below.
Position tab of a table data layer control
The Position tab lets you specify a table and a basic set of coordinates for the positions to plot. The details of how the data point at each of those positions is represented, including in some cases more coordinate values (e.g. for error bar sizes etc), are specified in the other tabs (usually Form).
Subsets tab of a table data layer control
The Subsets tab lists the defined Row Subsets for the table you have selected in the Position tab (see the Subsets window). The subsets All and Activated are always present; others may or may not be depending on whether any have been defined. Note that actions you take in the plot (for instance selecting a new subset by region) can result in new entries being added to this list.
The list on the left names the subsets with an activation checkbox and a grab handle; the panel on the right gives the detail for the currently selected subset. Select a subset to see/change its detail by clicking on it.
For each subset you can select:
/
) buttons at the bottom of the list
can be used as a convenience to make all the subsets visible or invisible
at once (which might be useful if you have lots of subsets).
Form tab of a table data layer control
The Form tab lets you control how the data coordinates
specified in the Position tab will be used to
generate graphics on the screen.
On the left is a stack of different Form types that will each be plotted;
a default item Mark is usually included to start with,
but you can add more from the 
Forms menu button.
Each form in this stack can be turned on or off individually by clicking
its activation checkbox, dragged up and down using its grab handle,
or deleted using the 
Remove Selected Form
item from the Forms menu.
When you select a form by clicking on it in the stack, a configuration sub-panel will appear on the right hand side of this panel. This allows you to select style information to determine the details of how the plot will look for each layer, and sometimes also reports information calculated by the plot. The details differ greatly between forms, but they generally contain two sub-panels for defining the style details, Global Style and Subset Styles: