Facts are real figures that are relevant for reporting. Facts can be classified in 2 different types. Homogeneous Facts and Heterogeneous Facts . Homogeneous Facts are the ones that connects to same facts in different tables, while the other one will map to one or more different facts from different table.
A fact expression can be Simple (Just using a fact column as expression) or Derived (using more than one fact columns in the expression).
Fact Creation Wizard
Fact creation wizard will enable us to create homogeneous simple facts (Basic facts), for heterogeneous derived facts , we may need to use Fact editor. Fact creation wizard can be taken during the project creation wizard step or directly from Schema menu of Microstrategy desktop. For quick creation of facts during initial project creation, we can also define rules for the facts, from the ‘define rules’ button on the wizard screen. Rules can be based on data types. We cannot use fact creation wizard to modify existing facts.
Fact Editor
In the Microstrategy desktop, select fact in the left pane and right click on the right pane , select new fact – this will load fact editor.
The column alias will define a temperory column name and data type for the calculated expressions. Microstratergy will use this defeniton while generating SQL. We can modify this to an existing column or by creating a new column alias using the ‘Modify ‘ button.
Bineesh 
This is a good breakdown on Heterogeneous Fact creation, and the need for them.