Professional Services firms tend to bill based on project scope. This means that the relationship with a PS firm requires careful scope management. Many IT Outsourcers will do project based work as well and when billing in this way this would apply equally to them and some PS firms will be billing by the hour and so the IT Outsourcing relationship would apply. In a project it is important that everyone be acutely aware of the scope and how it is defined. A large amount of overhead must go into the scoping by both sides as it is the scope document that will define the ability for profits and cost. PS firms are by necessity experts at ensuring that scopes are well defined and profitable to them. It is very easy for a naive IT department to improperly scope a project and be left with a project that they feel is incomplete. If scope management is, and you will excuse the pun, out of scope for your organization then it is wise to pursue Professional Services arrangements via a more flexible term such as hourly or time and materials.