Firstly, players who meet a certain threshold of league experience (based on a complex formula based on position, age, and time spent in the NHL) cannot be designated for assignment to a minor league club unless first placed on waivers.
Secondly, players who spend at least one season playing in another country's major professional hockey league are placed on waivers when returning to the NHL.
Thirdly, players who are released from their parent club unconditionally are placed on waivers before they become free agents.
Teams have 24 hours (measured from noon Eastern Time following the announcement of the transaction) to place a claim on a player.
If the player clears waivers, the intended transaction (either changing teams or outright release) proceeds.
Players in the National Basketball Association are placed on waivers if they are released by their parent club during the season.