Locales
XFDL forms are designed to be locale and language aware. This means that each form is designed for a specific language and set of locales. Locale support is identified through the xml:lang attribute. This attribute is primarily added to the XFDL tag in the form, and identifies which locale the form was designed for.
Base-64 and compressed encoding of binary data
In XFDL, option and suboption elements are allowed to store base-64 encoded and compressed base-64 encoded binary data such as signatures, images, enclosed word processing or spreadsheet documents, and so on. XFDL allows an encoding attribute to control whether an element contains data in a format other than plain XML character data. Both compressed and uncompressed base-64 encoding use no characters that are illegal in character data. Typically, based-64 and compressed base-64 encodings are used with the <mimedata> option in XFDL.
Scope identifiers (sid)
An XFDL scope identifier, or sid, uniquely identifies an element within the scope of its logical parent. Each <page> element must have a sid attribute that uniquely identifies the page within the surrounding XFDL form element. An item element must have a sid attribute that uniquely identifies the item within the surrounding page element.
Adding comments to XFDL
Comments are text added to the form that is ignored by XFDL processors. This allows form developers to document the form from within the XFDL source code. This helps subsequent form and application developers to immediately understand the purpose of a particular block of markup, such as a complex compute or function call. Comments are always wrapped in a special sequence of characters that indicate the beginning and end of a comment section.
Document reproducibility
XFDL processors are expected to preserve the XML prolog and epilog, the comments within the XFDL element, and all attributes appearing in start tags but not specifically defined by XFDL. The attributes must be associated with their respective start tags, and the comments must be associated with the respective pages, items, options, or array elements to which they apply. Additionally, all foreign-namespaced elements and attributes must be preserved. The XFDL processor must be able to reproduce these language components for signatures and for saving or transmitting the form.