Once security screening actually begins, it's not looking at the person. It begins by looking at the position and asking what level of screening is required for this position. It's then a question of gathering information about that person with their consent. As part of the security screening process, the individual is made fully aware of what information will be sought. Much of it comes from the individual. Some of it comes from other departments or external sources. The security screening official in the department who's hiring them then puts all of that information on the table and does a collective assessment, taking into account all the information available—its accuracy, its completeness and the particulars of the circumstances of that individual. They then make a call.
The individual is informed of that call. If it is a negative decision, then they are informed in writing as to why and exactly what their rights of recourse are. Post the granting of a clearance or a status, as security screening is not a one-and-done process, there's a continuous maintenance process to make sure that as the person goes through their career, regular recurring checks are done, depending on the level. That clearance is updated when necessary.