Several federal agencies maintain RDCs, in which approved users can access agency data that are not released to the public. The data never leave the site, and output produced from data held in the RDC cannot be removed without a disclosure review, which can take different forms. For example, RDC staff may be authorized to review output, or the output may have to be screened by an agency disclosure review board. The types of data manipulations allowed to RDC users are limited. Linkages between databases may be prohibited or restricted. Users may not be allowed to attach portable storage devices to the computers or terminals that they use, and even printing of output may not be permitted (one RDC emails output to users after it has been reviewed). Obtaining access to an RDC requires submission of a proposal, and acceptable uses may be restricted to applications that carry potential benefits to the agency. Some agencies impose additional requirements; the Census Bureau, for example, requires that its RDC users undergo a background check and obtain employee-like “Special Sworn Status.” The entire approval process may require several months. Despite these restrictions, RDCs are an important venue for access to data that agencies do not release on public use files. Reflecting demand, the Census Bureau has added several new RDCs in the past several years.