The Ball Collection Album (BCA) is a feature in Golf Clash whereby special (paid) balls are arranged into collections (known as Generation Albums) of 20 balls. Certain balls are excluded - these include any balls you can purchase with gems, certain licensed balls and some balls that were awarded as Tournament Prizes. To view your BCA select the golf bag icon at the bottom of the main menu screen > select the ball tab > select the album option at the top right of the screen.
As well as providing ball collectors with a way to view their inventory, the BCA was also introduced to enable players to unlock and receive a new ball called a Neutron ball. This is a Power 4, Spin 4, Wind 4 ball with Precision 4. You are awarded 5 of these ball when you own (or have owned) all 20 balls within a single Generation Album. More are awarded as you increase the Prestige level of all of these balls to certain levels.
Realistically, unless you are a very regular buyer of paid balls (and have been for some time) you are unlikely to own all 20 balls in a specific album. Currently there are 2 ways to obtain a ball you did not purchase at initial release - via a Ball Prism chests (if you have maxed out most or all of your clubs), or by purchasing new Generation Chests. These chests currently cost between $5 - $10 and award you 4 different balls (5 of each for a total of 20 balls) from a specific generation. They may seem a lot like loot crates, which is because that is exactly what they are! If you only require 1 ball to complete a Generation Album, there is absolutely no guarantee that you will receive it by buying a Generation Chest. You could spend hundreds of dollars chasing something that you never unlock.
Each ball in your inventory is marked with a specific Prestige level. Broadly this relates to the number of times you have used that ball - the more you use it, the higher the Prestige level. The BCA also enables players to trade in balls they have bought but no longer require for tokens that can be used to boost the Prestige level of other balls in that same Generation Album. Furthermore, you can exchange surplus balls in your stock for Generation Tokens.
All in all, this entire feature is extremely over-engineered and is clearly designed to drive sales and boost revenue. Sure, it essentially allows players to go back and (maybe) collect some balls that they did not buy when first available. The Neutron ball is a nice ball too, very versatile stats and an excellent Precision 4 attribute, but there have been balls sold in the game with very comparable stats (and will undoubtedly be again).