You can set up Workflows and make Decisions on either the user-level or order-level depending on what criteria you want to look at and which entity you want to take action. For more on how to create Decisions, please see this tutorial.
Different criteria are available at different levels. At the user level, for example, you can use Sift Score, shipping/billing addresses, email addresses, and any custom fields you may send. At the order level, you can use features like CVV status, currency code, or item ID.
Which level should I use?
When thinking of which level to use, it's best to think in terms of what the Decision is going to be made on. For example, if you want to cancel, hold for review, or automatically accept any order, you will want to set up an order-level Decision in your Workflow. If you want to ban, manually verify, or whitelist a user, you will want to set up a user-level Decision in your Workflow.
When creating Decisions you can toggle between User and Order levels using the icons near the top of your screen: