Just reading some of these replies, I think some people are talking about proffer to displace and others are talking about displace. If you proffer to displace, the company does not displace junior pilots. So if you want DFW 175 but have a proffer to displace DFW 175 followed by DFW 145, you could be awarded DFW 145 if no open DFW 175 spots were open. Then, when they ran the displacements AFTER that, you would not be displaced again since you are already captain at that point, and therefor would not be displacing junior pilots.
Remember
1) Vacancy bid is run first
2) Proffer to displace is run next
3) Displace is run last
Proffer to displace and Displace are treated very different during the bid run. AND, if you leave your 3P blank, do not Proffer to Displace, and the company displaces all qualified FOs as they have been doing, you WILL get what your seniority will hold if you have your 3D set correctly.