Open the playlist, go to the Screens tab, and choose Select screens.
The dialog offers both options, and states the choice plainly: select individual screens, or assign a label to include every screen that has it.
Screens or labels
Individual screens targets exactly those, and nothing changes unless you come back and edit it. Right for a handful, and for a playlist that genuinely belongs to one display.
A label targets every screen carrying it, now and in future. A screen given that label next month starts playing without anyone opening the playlist.
Every screen also has a label matching its own name, created when it was paired. That is what makes targeting one specific screen work, and it is why renaming a screen does not break an existing assignment.
Once you are past a handful of screens, labels are the answer. See Labels.
Seeing what it reaches
The Screens tab lists every screen the playlist reaches, however it reaches them, so a label assignment and a direct assignment look the same here. That is the quickest way to confirm an assignment did what you meant, rather than what you typed.
More than one target
A playlist can carry several labels and several direct screens at once. It plays on any screen matching any of them, so the targets add up rather than narrowing each other.
Removing it from screens
Remove the label or the screen from the Screens tab and save.
Those screens stop playing it on their next sync, and clear any content files no longer needed by another playlist. Nothing else is affected: the playlist itself is untouched, and every other screen it targets carries on.
When it does not arrive
In this order: is the playlist enabled, is its schedule active right now in that screen’s local time, does the screen carry the exact label including capitals, and has the screen finished syncing. See How a screen decides what to play.