Opaque cache? Invalidation? Need someone explain in detail


This is kinda dumb question but need someone who can explain about meaning of opaque cache and its invalidation from Slate Architecture page on docs page below.(Understanding the Slate UI Architecture in Unreal Engine | Unreal Engine 5.0 Documentation)

I’m currently reading the articles and papers about MVC, MVP, MVVM and IMGUI to dig into slate system in engine. Epic applied good features from different pattern of UI design on their slate system and seems to me(if what I understand from documentation is correct), polling and notifying bewteen model and view, procedural UI generation and other good things are integrated on it.

Anyway, my question is what is meaning of opaque cache and invalidation for its state? It comes right after section “Core Tenets” on Slate Architecture page which I linked above.

Hope someone who can understand my short english and brings me the right direction. Thanks!