FormFiller/FormChecker is a web application for working with Grants.gov application packages.
At the beginning of the application lifecycle, one would visit FormFiller to get an application pacakge with key information (e.g. contact information for the Authorized Organizational Representative and contact information for the Principal Investigator) pre-filled. It can also modify the behavior of PureEdge forms; for example, if the application is for NIH, the NIH Commons ID is made mandatory on the R&R Key Person Profile form.
Before submitting the application to Grants.gov, one would run the package through the FormChecker to validate it against rules that are specific to the sponsoring agency or the applicant institution.