Rentals: Players that have one year or less (in-season) left of control. Some players with 2nd year options are included, but where those options are likely to be declined, making the player a more likely one-year rental
Positional Surplus: Players that are on teams where they become expendable due to major-league ready/near-ready players that can step in and be productive in the same position. Refer to the MLB & MiLB Radar to see who those players may be.
Non-Contending Team: Players on teams with little-to-no chance of making a run for the playoffs, so valuable players become expendable to acquire younger, cheaper players that give the non-contending team a better chance to win in the future.
Salary Dumps: These are players where the salary outweighs the performance. Usually these players are not traded unless there is money coming back in a trade, or a prospect is paired with a high-salary underperformer.