Blizzard normally supports a maximum of five active boss frames, although a handful of modern encounters have many more active bosses.

kExtraBossFrames provides up to eight boss frames when using a supported addon.

Currently supported addons are:

• ShadowedUnitFrames
• ElvUI: (Deprecated) ElvUI v12.72 adds built-in support for boss frames 6 - 8, making the kExtraBossFrames overrides obsolete. If you're using ElvUI for your boss frames, you no longer need kExtraBossFrames installed.