UAT in DoD
The moment a Product Backlog item meets the Definition of Done, an Increment is born
Ideally an increment should be ready enough to release anytime within or by end of the sprint. As UAT is one of quality standards before release, UAT also should be added within DoD for all items. Though it is not possible for all products or services, If this is followed in any team, How the estimation of stories are happening ? Does developers include UAT person inputs during estimation process or in sprint planning ? If UAT is in DoD and not considered during estimation, Doesn't it create problems for developers to forecast better in a sprint ?
I don't agree with the assertion that a UAT should be added to the team's Definition of Done. UAT is user acceptance testing and is performed by people who use the product. In most cases, the team building the product is not the user. Even in the cases where the team does use the product, they may not be fully representative of the broader set of users and not in a position to complete user acceptance testing on their own. If UAT is included in the Definition of Done, then the team has an external dependency in order to get work to a Done state. This means that, in any given Sprint, large amounts of work could be not Done because the external dependency was unable to do the work for any reason at all.
The team's Definition of Done should be such that downstream activities, such as integration or user acceptance testing, complete successfully. If the result of user acceptance testing is that the product is not acceptable, the team should understand why and make changes to their way of working to regularly create Increments that are likely to be acceptable. Any other feedback from UAT can be treated like customer or user feedback and ordered with the rest of the Product Backlog.
By removing external dependencies, you no longer need to worry about how to estimate or plan for these external dependencies during refinement or Sprint Planning events.
I'll also add that it may not be desirable to remove all external dependencies from the Definition of Done. In some cases, the better answer may be to find a way to get the external dependency within the control of the team. However, I don't see this being a viable solution for many teams in the context of UAT.
UAT also should be added within DoD for all items.
My advice is to think less of UAT as something added to a Definition of Done, and more as something that ought to be assured by it. User acceptance is not an occurrence but a risk to be managed.