Creating an Analysis from Multiple Subject Areas - Using Measures Column from each subject area?

Summary

Creating an Analysis from Multiple Subject Areas - Using Measures Column from each subject area

Content

I don't understand the logic of when creating an OTBI Analysis from multiple subject areas why you need to add  a measure column from each subject area in order for the report to produce output ? Why is this needed please ?

Also, I under stand the Dimensionality Checkbox in the Advanced tab (" Show Total value for all measures on unrelated dimensions.") should be ticked only when you are using two folders in seperate subject areas which there is not database join between their tables ? Is this correct ?