Joined: Jan 2018
Not sure if this is 100% accurate or not but a quick google search got me this.
"A player can be placed on the Non-Roster list for reasons other than injury (birth of a child, attending a funeral), at the Commissioner's approval. His roster spot can be replaced, but the cap hit of both he and his replacement will remain. In cases where a player returning from IR or being picked up via waivers/trade would put a team over the 23-man limit, they can place another person on waivers and he'll be designated as Non-Roster until he clears & is sent down or is picked up (and if he does clear, he must be sent down). During this 24-hour period, both cap hits will count.
Again, the Commissioner has the authority/responsibility to make sure teams aren't using this section to cheat."