Microsoft online services, which are growing in popularity, are a subscription-based model and are very specific in its use. Licensing is much simpler to decipher, especially due to the lack of hardware associated with the service. Just like any other web-based service, you can buy one or multiple licenses, or a company-wide unlimited usage plan. For certain products, such as Forefront Protection for Exchange, you can purchase it in either a per device or per user subscription, dependent on your Exchange licensing model.
Online services are great for companies that need a Microsoft product, but don’t want to commit to purchasing them or, for example, will only need to use the service for a limited time. The subscription-based model makes licensing simple and is great for companies on the go, because they can be accessed from anywhere.