What do the letter icons represent in the graph styles manager?

Graphs are composed of collections of graph objects, that is, they are collections of labels, axes, fit lines, point markers, etc., as well as separate added objects such as circles, text boxes, or arrows. Graph styles address the properties of graph objects (e.g., size, color, thickness, and pattern of lines; size, shape, and color of point markers; colors and patterns used for definition of areas; size, color, and fonts for labels, titles, and scales; etc.). Graph styles are not used to add objects to graphs, but to specify the properties of objects already added to or specified for the graph. Styles can also be used to specify properties of objects that can be added to a graph later (e.g., arrows, rectangles, text, etc.).

Attributes and properties
There are two general classes of items for which a style can be created: attributes and properties. An attribute (designated with an ) is an object that affects the simple appearance of the graph, such as colors, line patterns, font sizes, font names, etc. A property (designated with a ) of a graph is an aspect of the graph that is not directly visible, such as what kind of plot to make, or what scale type to use.
Style collections
There are two additional classes of styles, collections of attributes ( ) and collections of properties and/or attributes ( ). All the elements of a collection of attributes are simple attributes, . All the elements of a collection of properties and/or attributes are either properties or collections of attributes,   and  .
User-defined styles
Any type of style that you create and save into your system will be denoted with a pencil on the icon (e.g., ), whether it is an attribute or property.