In other words, what’s the difference between “Unsubscribing” from one of your friends on Facebook or adding them to your Acquaintance list?
Subscribing to someone is a way of seeing their Facebook updates in your News Feed without being friends with the person.
While you don’t have to be friends with someone to subscribe to them, you do automatically become subscribed when you become friends.
Once you are subscribed, you further limit what you see from someone by mousing over one of their content items that appears in your News Feed and selecting the down arrow that appears on the upper right of the post. You can choose to see “All Updates” or “Most Updates” or “Only Important”. The default is “Most Updates”.
Facebook’s algorithm determines the precise meanings of each of those terms. “The news feed algorithm uses several factors, including: how many friends are commenting on a certain piece of content, who posted the content, and what type of content it is (e.g. photo, video, or status update),” according to Facebook.
Adding someone to your Acquaintance list automatically changes the subscription setting for that person to “Only Important.”
Selecting “Only Important” does not automatically add a friend to your Acquaintance list.
Acquaintance lists can also be used to manage which of your posts are seen by the members of that list. When you post a piece of content on Facebook, you can choose to share it with “Friends except Acquaintances”.
Facebook says that “people on your Acquaintances list will rarely show up in your news feed,” so it would seem to me.