When you designate a feature as “Additional” it creates an “Optional Subclass Feature” that the player can choose “in addition to” their regular subclass features. (Like the Optional features from Tasha’s Cauldron.) There’s nothin else to choose. If you instead want to create an Optional Subclass Feature that replaces a standard subclass feature you need to instead designate it as a “Replacement” feature.
When I select it, it doesn't say how to select a class feature and The box under it doesn't become available so how do I do this?
Do you mean in the homebrewer?
Creating Epic Boons on DDB
DDB Buyers' Guide
Hardcovers, DDB & You
Content Troubleshooting
yeah
When you designate a feature as “Additional” it creates an “Optional Subclass Feature” that the player can choose “in addition to” their regular subclass features. (Like the Optional features from Tasha’s Cauldron.) There’s nothin else to choose. If you instead want to create an Optional Subclass Feature that replaces a standard subclass feature you need to instead designate it as a “Replacement” feature.
Creating Epic Boons on DDB
DDB Buyers' Guide
Hardcovers, DDB & You
Content Troubleshooting