The link posted above will explain in detail, but the short answer is that the license is based on how many users you have. If you have 150 mailboxes, but only 100 users, you would only need an 100 seat license.
Edit: I neglected to read the post title and went straight to the content and saw that this is unrelated to the Blocker product, please disregard my post as it does not apply to the SA540.
