Used for: setting a field to a certain value.
With the "set to value" rule you can easily add missing information to your product feed or change the information that is already in your product feed. For example, for a Google Shopping feed you will need to add a gender field, which is set to either "male", "female" or "unisex". You can use the "set to value" rule to easily add this information.
The screenshot below shows you how:
With this rule, the "gender" field will be set to "male" for all of your products of which the product type contains "men".