There are six tournaments on the PGA Tour schedule without a cut: the WGC Dell Technologies Match Play, the Sentry Tournament of Champions, the Zozo Championship, the CJ Cup, the BMW Championship and the Tour Championship.
Several of them lacks an alternate list -- you're either in or you're not.
These no-cut tournaments come with guaranteed prize money for players able to compete, and most take up the PGA Tour on the offer. However, there are times when players are unable to compete because of injury, personal issues or other medical problems that require their attention.
The PGA Tour has policies in place for these cases, though they differ depending on whether or not the tournament is part of the World Golf Championships.
For non-WGC events, like the Sentry Tournament of Champions, BMW Championship and Tour Championship, where there is no alternate list, a player suffering from injury or personal emergency can receive unofficial last-place prize money (and no FedEx Cup points) without having to show up and register for the event. If the player shows up and then withdraws anyhow, the same issue applies.
For no-cut events with an alternate list, a player will be replaced in the field if they withdraw ahead of time, as is standard practice.
For World Golf Championships events, players who withdraw before hitting their first shot in the tournament are unable to collect any prize money for the tournament -- official or unofficial. They are able to donate their share of the prize money to the charity of their choice. However, if a player is able enough to hit just a single shot in a WGC tournament and then withdraw, they can collect last-place unofficial money.
The PGA Tour wants to give its players the financial compensation that comes with earning spots in these exclusive events, but they will not allow players to advance on the money list or FedEx Cup points list without actually attempting to compete in said tournaments.