I was working on my first deal as a young lawyer. I represented the buyer. The lawyer representing the seller drafted the purchase agreement and emailed it to me for review and comment. I revised the agreement with my requested changes and emailed it back to her. She phoned me. She was furious because I had failed to "blackline" my changes to the revised agreement. I didn't know what she was talking about. No one at my law firm had told me about this. Opposing counsel schooled me on the custom and courtesy of blacklining that day, and it's a lesson I've never forgotten.
"Blacklining" means marking your additions and deletions to a document so other people reviewing and working on the document can easily identify those changes. When I was a summer clerk at a law firm before word processors were prevalent, we actually blacklined by hand-marking documents with a black felt tip pen and ruler. It was tedious work. Now, blacklining typically is done using the Track Changes feature in Word. It's also possible to create a blackline draft using the Compare feature in Word.
Business lawyers customarily blackline their changes to documents for at least two reasons. First, it's a professional courtesy to others working on the documents, especially opposing counsel, so they do not have to re-read the entire document each time a change is made. This makes negotiations more efficient. Second, it implies that the unchanged portions of the document are now agreed upon, which reduces the number of open issues to be negotiated.
Blacklining brings order to the deal process. It makes the work on the deal documents more efficient. Negotiations proceed smoothly.
Failing to blackline is rude and breeds distrust. It's counter-productive. Don't be that person.