These file extensions have to do with plot style tables. A plot style table is a collection of plot styles (think pen setting) assigned to a layout or the Model tab. There are two types of plot style tables: color-dependent plot style tables and named plot style tables.
Color-dependent plot style tables (.ctb) use an object's color to determine characteristics such as lineweight. Every red object in a drawing is plotted the same way. While you can edit plot styles in a color-dependent plot style table, you cannot add or delete plot styles. There are 256 plot styles in a color-dependent plot style table, one for each color. Is is the method that LMHT Associates and most architectural firms use.
Named plot style tables (.stb) contain user-defined plot styles. When you use a named plot style table, objects that have the same color may be plotted differently, based on the plot style assigned to the object. A named plot style table can contain as many or as few plot styles as required. Named plot styles can be assigned to objects or layers, just like any other property.
The use of either .ctb or .stb is a drawing variable. If a drawing has been created using the .stb format you cannot plot it using .ctb format without converting it first. See How to section on changing a drawing to use .ctb from .stb.