The Nintex Workflow Set Item Permissions action allows you to give certain permissions to users for the current item the workflow is running on. The out-of-the-box action usually shows you the normal permissions:
But there are occasions when you need more specialized permissions.
For example, what if you want to give some access to view the list but to also to add alerts. In this case, you can create a custom permission level.
You can do this by going to Site Settings > SIte Permissions > Permission Levels (in the ribbon) and then "Add a permission level".
Once you have done this, go back to your workflow, reopen the Set Item Permissions action and in the permission drop down, you'll see this:
Now you have your new permissions level. This requires no special coding or special workflow design. The Set Item Permissions action simply dynamically picks up the available permission levels that you have have in SharePoint.