Hi Wen, this is my understanding and it may help you - folks please correct me if i got this wrong:
Understanding by ANALOGY - birth certificates are like your block table record
Imagine you have a really important document like your GREEN card/birth certificate etc. you have to keep this document locked away in your solicitor's office. this green card is exactly like a block table record. you only have one of this document. but let's just say you want to have many copies of your green card. Are you going to recreate a green card (and have many originals) so you can give one to the IRS, another to the department of immigration, another to the department of homeland security etc etc? No! Of course not: you are not going to recreate it: you are going to make photocopies of it. It makes sense because photocopies are cheap and easy and don't cost (much) memory.
The photocopies of your green card are like lock references. you can have many photocopies of your green card, and you can paste them anywhere you want: on the street, in your home etc. you can even delete them. you can delete or destroy all of them, but you will still have a copy of your green card located safely somewhere in your house, or your lawyers office. If you destroy the original green card then guess what - you will no longer be able to make copies of it (i.e. you won't be able to make any block references if you don't have a block table record which it points to.)
summary:
You have (1) block definitions - also called block table records, and then you have: (2) block references. Block references and block definitions are similar but related things.
The block definition is in the block table. This does not appear anywhere in your drawing. if you want to put it in your drawing then what you do is: you create a block reference (which points to the block definition) and you then put the block reference into the model space.
hope that helps.
BK
The block definition is in the block table. This does not appear anywhere in your drawing. if you want to put it in your drawing then what you do is: you create a block reference (which points to the block definition) and you then put the block reference into the model space.
Or in a PaperSpace or in any other BlockTable(Record) -- Nested Blocks.