It must also store information that is specific to the interface involved. Sometimes this information will be visible in the drawing, and other times it will not be visible. Visibility depends on the application and nature of the data. There are a couple of ways to store information in a drawing that meets these needs. One way is through the use of block attributes.
Attributes are independent entities attached to block inserts.
They can be used to hold text in a visible or invisible fashion. As a result, they offer a wonderful way for AutoCAD to interface with other applications. In order to facilitate the access of attributes by external systems, they are tagged, which means that the attributes contain not only variable text information but also have tags or names attached to them so that they can be extracted into an external text file for manipulation by another system using a common reference. For example, suppose you had a library of blocks that represented standard building objects such as windows, doors or bolts. For bolts, the tagged text might contain sizing information.