For inherited process model in VSTS, you can use Custom Rules to define rules that can clear the value of a field, copy a value into a field, and apply values based on dependencies between different fields' etc. You can define actions against conditions using custom rules. This provide you the flexibility in VSTS to go beyond the level of setting a default value or making a field required. To customize work items in an existing team project of Hosted XML model for VSTS, or on-premises TFS, you have to customize, downloaded process template or individual work item XML files (using witadmin tool) respectively. Differences between process models are described here.
Let’s look at how we can use the custom rules in inherited process template. In your inherited process template you can go to a given work item type, by clicking on the name of it as described in here. Then you can see a tab Rules, where you can define the custom rules for the work item type.
At present there are some limitations in defining custom rules. You can use only maximum of two conditions with an AND clause. There is no OR condition. And conditions are limited to predefined set of condition types.
But this is awesome flexibility for VSTS templates and this will get more improvements in future hopefully.