Following is an excerpt from AutoCAD 2002 Help --> Support Assistance --> Welcome --> What's hot page.I found it interesting.
The Block Attribute Manager is used to edit Block Definitions, and the Enhanced Attribute Editor enables you to edit the attributes in an individual block.

In the Block Attribute Manager, you can edit block definitions as follows:
  • Changes are applied to all existing instances of the block as the block definition is edited (Apply Changes to Existing References is selected)

  • Changes are only made to block definition, and will be applied to future instances when they are inserted (Apply Changes to Existing References is not selected)

  • Attribute values (such as Tag, Prompt, and Default) may be changed, but the values assigned to the attribute values in individual blocks may not be edited in Block Attribute Manager. For example, if you change the Tag "COST" to "UNIT COST", the value of 19.99 which you applied in a specific block to the COST value is not altered. You will have a UNIT COST of 19.99.

  • In the Block Attribute Manager you use the Move Up and Move Down buttons to change the prompt order of the attributes.
In the Enhanced Attribute Editor, you can edit individual blocks as follows:
  • Changes are applied to the selected block instance only, and cannot be applied globally.

  • Attribute values may not be redefined, but the individual value applied to the attribute may be changed for that block.