Know or know how to work out the absolute minimum that you are prepared to settle for in a negotiation.
The minimum or bottom line is usually a long way below what you expect to achieve from a negotiation. A true bottom line is where the point is reached when it is not worth negotiating because there is no conceivable benefit in doing so.
The knowledge named 'Know bottom line' is considered to be quite critical.
In addition, the knowledge is thought to be quite simple and quite easy to understand, it is mostly learned by study, and is mostly specialised knowledge.
Opinion is that it is slightly justifiable knowledge.