Recently I had a requirement where i had to populate the value of a field depending on other fields. To achieve this i used post default value with a conditional expression, the exact requirement was something like.
1. If type of transaction is Redemption and Sub type is Product then set the field value Product Name to value say Sample Product.
2.else set it to null.
The solution can also be achieved by scripting but its more easy with configuration.
We made a expression that validates the values of type and subtype and the sets the value of product field.
expr : iif( [Type] = "Redemption" AND [Sub Type] = "Product" , "Sample Product" , "")
Add this expression in post default value and u r done.