This feature enables you to update a given field or set of fields with specific values upon successful assignment by SRR. It is set at the MatchGroup level.

Example Scenario: Update 'Rating' field to 'Hot' when the record is assigned by the Hot Leads MatchGroup

In this simple scenario we will look at how the Rating field on the Lead object can be updated with the value Hot when a record is matched and assigned. Imagine we have two MatchGroups, one that matches regular leads and another that singles out the HOT leads:

All we have to do in order to set the Rating field to Hot (and we are also going to set the Description field as well), is use the Update Fields on Assignment setting in the Optional Setting section on the left.
The syntax is: "firstFieldname":"firstFieldvalue" with further fields separated by a comma, e.g. 

"Description":"Hot Lead Matched by SRR","Rating":"Hot" 

So for our example, it will look like this:

In summary - this will set the Rating field (and the Description field) to their values stated above every time a record is matched and assigned by this MatchGroup.