Hi,

If you have a block in the drawing file "A" and you can see this block in Content palette - this mean that the block definition stored inside the drawing "A".

You can edit it in Block Editor and changes will apply to the all insertions of this block in the current drawing "A".

If you have a block with the same name in another file (or have DWG file with the name which matches the name of the block you have already in the first drawing "A") and then you want to insert this external block (or whole DWG file) to the drawing "A" from content palette or with INSERT command - AutoCAD should warn you with such message:

If you will choose "Redefine block" option - block definition and all its insertions in drawing "A" will be substituted with block contents of the "external" block you are trying to insert.

Maxim