The editor maintains another buffer in parallel with your text buffer called the line delete buffer. This buffer has the same structure as your text buffer, however, it can not be displayed or directly operated on by the editor's many commands. Each time you delete a line via the Delete (d) command it is moved from your text buffer into your line delete buffer. You can restore deleted lines using the special forms of the Append (a) and Insert (i) commands which can move lines from the delete buffer back to your text buffer.
The moving of lines between the two buffers is slightly more complicated than is indicated above and is best explained by an example.
If you were to delete 5 lines, one at a time (say via the F3 key) it would be nice if you could undelete them one at a time so that the last line deleted was the first line restored. This is particularly nice when you delete one line too many and just want to restore the last one, not all of them. Conversely, if you were to delete a group of 100 lines as a block (say via a tagged delete) you do not want to have to restore them one at a time but want them restored as a block as well. The above two scenarios describe, from a user's point of view, the editor's implementation of the line delete buffer. Associated with the buffer is a flag which indicates whether the buffer contains a series of single line deletes or one block delete. To avoid confusion, the editor will purge the line delete buffer before adding in the following circumstances.
Put simply, if you delete lines one at a time, they are undeleted one at a time and if you delete a block of lines they are undeleted as a block. Mixing blocks or types is prevented by purging before adding, if necessary.
When working with a very large buffer it is possible for the editor to run out of memory. When this happens it will purge the delete buffer in an attempt to free up some space. You will be warned of this by a message on the command line which you must clear (like an error) by typing a carriage return. Deleting all lines in a file, then attempting to edit another large file will often generate this message. In this case you have all of the original file in memory in the line delete buffer and are trying to read another large file into the text buffer. They may not both fit!