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Figure A-1.  Classes of Geometric Shapes

All squares belong to the same class because they all have similar qualities, 
such as four equal sides. The term instance refers to a specific instance of 
a given class. For example, a blue square of four inches per side would be 
one instance of the class square, and a red square of five inches per side 
would be another instance. The term object is often used as a synonym for 
the term instance, although in some contexts it might also refer to a class.

Each class defines a set of attributes which represent its externally visible 
characteristics. The set of attributes defined by a class is common to all 
instances within that class. For the class square, attributes could include 
length of each side and color. For the class circle, attributes could include 
radius and color. Each class also defines a set of services (or methods) 
which is used to perform an operation on an instance. For the class square, 
services could include resize, rotate, or change color. 

Figure A-2 illustrates the object modeling used within the DeviceNet 
Specification.

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Class Square

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Class Triangle

Class Circle

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