What is the difference between Agency and Screening Group?

Historically, AQS used an abstraction called "Screening Groups" for security access control - i.e. who was allowed to modify data in AQS.  Under this model, Monitors were owned by Screening Groups and users were assigned to screening groups based on which monitors they needed to access.  This security model has at least two problems:

1. Screening Groups do not relate well to anything in the real world.  They are also not mentioned in the "Definitions" section of 40 CFR Part 58, because of this, they were (are) a point of confusion for training new users and explaining what they are and how they are used.
   Screening Groups do not provide a fine-grained degree of access control.  For example, if a monitor is owned by a Screening Group, then when a user is granted membership in that Screening Group to submit raw data, they also have permissions in AQS to modify the metadata for that monitor (e.g. Monitor Type, Sample Period, Required Collection Frequency, and etc.).
2. Agency Based Access Control:  It was observed when the Monitoring QA Project was initiated in 2013 that 40 CFR Part 58 Section 1 = "Definitions" describes the types of real-world agencies that relate to the ambient air quality monitoring program, and the roles that they play in this program.  It was further observed that many of these roles, for example" Reporting Organization" and "Primary Quality Assurance Organization" are already defined in AQS for sites or monitors, but only used as metadata and not for any other purposes in AQS.  Also, at about this time, AQS Federal staff started receiving requests from Regional AQS staff to allow users who were assigned to an agency that had a role defined for monitors, e.g. Reporting Organization, to perform that role (submitting Raw Data) for the monitors.

Because of the above, starting in 2013, the process was initiated to define each of the Agency Types in AQS that are defined in 40 CFR Part 58 Section 1, and to utilize those agency types and roles for access control in AQS as the preferred access control mechanism.  However, most of the user-interface forms in AQS, e.g. Maintain -> Monitor, had been programmed to be so tightly integrated with the Screening Group model that they could not be decoupled from screening groups without being totally redesigned.  As a result, presently both Screening Groups and Agencies provide access to AQS data.  The newer user interface forms, e.g. those for maintaining QA data, only utilize Agency based access control, while the older forms provide both or just Screening Group access.  The AQS Batch process has been updated so that Agency access control applies to all transaction types, but historical Screening Group access is also allowed to all transaction types except QS.