Typically, there are two giant buckets people fall into when they are looking at federal contracting...IT and non-IT. The IT contracting space is much larger and has it's own section of categories and job descriptions. The reason it is important to know the difference is because the titles between some types of roles will be exactly the same however the content within the resume and job requirements will be different. Take note of what you are applying for and be mindful of the language used on the posting. Sometimes they will post the exact category they are leveraging and mention this in the posting. Not a bad clarification question to ask the recruiter you are working with.
If it is an IT related project then you are likely going under, what we refer to as, a TBIPS contract. Task Based Informatics Professional Services (TBIPS) are IT related contracts specifically geared towards technical projects or for resources that are not technical but associated with a technical system in one way or another.
If you are non-IT related projects, you fall into the TSPS (Task and Solution Professional Services) categories. Pretty simple test is this: Are you working on an IT related project? If Yes, TBIPS, if No, TSPS. While these are broad generalizations, they are not true in every case, this is intended to speak to the vast majority.
To be clear, this article is really intended to those interested in getting into the contracting field as this is applicable to the kinds of contracts that will be leveraged to be able to pay you your hourly/daily rate.
Both of the above listed categories pay the recruiting firms a per diem depending on the contract vehicle and the department, each one is uniquely tendered. Under normal circumstances, they are only on a T&M basis instead of done as a deliverable based project. This is doable, but needs someone to explain it.