The Design and Reference Guide provides detail on what a Finance Function Type is and the different types of Finance Function Types
Design and Reference/Financial Model Guides/Finance Function Types.htm
In order for many of the Finance Function Types to execute, the Finance Business Rule needs to be assigned to the Cube under Cube > Cube Properties > Business Rules. This will execute upon running the calculation sequence associated to a Cube(s). As mentioned by ckattookaran, the CustomCalculate Finance Function Type executes differently. This Finance Function Type is not considered during the Cube execution. The logic for the CustomCalculate Finance Function Type is created in a Finance Business Rule. However, the function is executed through a Data Management Custom Calculate step. It is here where you define the Finance Business Rule where the Custom Calculate function is located, define the Custom Calculate Function name, and any parameters to be passed during execution.