Choosing a different stack for a workflow from what you set for the app will give you some handy new tools.
Until now, whenever you selected a stack in your app settings, that stack was used no matter which workflow a build used. For certain cases, this was not convenient at all, since you might want to use one stack for a certain workflow and another stack for an other. From now on, you can achieve this with our per-workflow stack selection.
You can now select the stack you prefer for each workflow, and (if a build uses that workflow) the stack which is set for that workflow will be used instead of the one set for the app.
So for example you have a project configured in a way that it uses a stack with an older Xcode version and you would like to update it. You can now create (clone) a workflow which uses a stack with a newer Xcode version, run a few builds with this workflow - it will use this stack - and when you are sure your code is ready to run on the new stack, set it as the default for the app, making it used for all workflows which are not defined otherwise. But until you are 100% sure it'll work on the new version too, you can keep the old one without too much hassle.
By default, all workflows’ stack is set to default, meaning it will be always the same as the stack set for the app - whenever the app’s stack changes, these workflows’ stacks will change as well.
If you set the workflow stack to be the same as the one for the app (but not by selecting the ‘default’ option, but the exact version), when you change the app's stack to another (let's say the latest), the workflow's stack will stay unchanged, that is the previous version.
All the above described, along with the app’s stack configuration can be managed in the Workflow editor, under the Stack tab.