I am developing a randomized clinical trial with women substance abusers on probation or parole. I want to answer the question if identifying information will be used and why.
In general terms, you will need to keep identifying information on each participant for the purpose of obtaining their data and following up with them. This will need to be kept secure usually on a database that only the research team have access to.
Each participant is usually given an id number linked to their identifying information so that any data collected such as questionnaires or interviews will only show this id number rather than their identifying details. Whoever is in contact with participants would need to be able to identify who they have approached, who has responded, who they need to follow up etc. But the data that is collected from each person should not be linked to that individual, only the number that they are given so that number would appear on the top of questionnaries that they complete so that personal details can be separated from any data collected for analysis. It would be important that you protect their right to anonymity and confidentiality from the research so that others do not know that they are taking part unless they are ok for that to be revealed, for example they consent to take part in an interview or consent for their information to be made available to others.
I'm not sure about the US system specifically but trials and generally very tightly controlled and there are guidelines for their conduct such as the Office for Human Research Protections (OHRP) which would be able to help you identify how to treat different sorts of information. I'm not sure if this answers your question because it was quite broad but if you have anything more specific please let me know.