When teams trade a player, they can retain, or keep, some of the players' salary and cap hit.  The team keeps a % of both the cap hit and salary for the remainder of the contract. 

Therefore, if a Player with a Salary of $2.0M and a Cap Hit/AAV of $3.0M is traded with 20% retention, the trading team would continue to pay 20% of the $2.0M Salary and would continue to have a cap hit of 20% of the $3.0M Cap Hit.

The maximum retention % is 50%.

Teams can only carry a maximum of 3 Retained Salaries at a time.  

An individual contract can only have a maximum of 2 teams retaining salary on it.

The maximum amount of retained salary by a team is 15% of the Salary Cap for the current year.

If a team retains salary on a traded player and that player is later sent to the minors, there is no change to the cap hit for the retaining team. 

If a team retains salary on a trade, they cannot reacquire that player for one year from the date of the transaction, unless the contract ends prior to one year.