Using a Nested IF statement within a CASE Picklist function can allow you to set specific parameters on certain values. In this example, we are creating a Category field to determine the Category of Marketing based on primarily Source Type but also sometimes by Source Name.
When the Type is "BCI" we want the formula to return "Partners" when it is "Home Depot" we also want it to return "Partners" and this continues for the next several values in the Type field.
We reach an exception for Showroom. If the Type is "Showroom" we also want to consider the value in the Source Name field to help determine the Category. If the Name is "TV" we want the formula to return a value of "TV" however, if the Type is Showroom and the Source Name is "Billboard" then return "Billboard" and finally if the Type is Showroom but the Source Name is anything else (besides TV or Billboard) then return a value of "Events" in the Category field.
For more on this topic, see also: Salesforce Formula Functions and Operators