The Extensible Dimension inherits and extends Dimensions to give Business Units, Scenarios, and Project Phases the level of detail they need. This also includes operational independence, reporting needs, etc. With the Cube requirements, data is loaded into one cube and is visible in another.
Some companies have multiple businesses or business units that do different things. Companies can manufacture different products, have different processes or even different requirements. Because of this, different parts of businesses may need different dimensions either by project phase or scenario (i.e Actual vs Budget vs Forecast vs Long Term planning).
A chart on what horizontal and vertical extensibility contains.
You are able to extend any Account Level Dimension such as: Account, Flow, and UDs. It is also based on Summary and Detail members either by Cube or Scenario. Summary Members is applied in the Parent Dimension. Detail Members is applied in the Child Dimensions.
The relationship between Summary and Detail members is then applied to Scenarios. You can extend Scenarios whenever you want to control what Scenarios are available for each Cube. You can do this by Scenario Type.