From Riftui Wiki
RIFT requires a UI context to be created in order to create any frames.
All frames must have a parent frame; a context is the "main" parent frame.
If you use multiple frames in your addon, you may want to create separate contexts for them.
Use UI.CreateContext() to create a context.
Context inherits all members and events from Frame.
-- Create a context with the name "SampleContext" -- local context = UI.CreateContext("SampleContext") -- Create a frame parented to this context -- local frame = UI.CreateFrame("Frame", "SampleFrame", context)
-  - an example of situation which requires multiple contexts.