There are several levels of nodes in an XFDL form: form (or root), page, item, option, and argument (which can have an infinite number of levels). Each node has four properties: literal, type, identifier, and compute. A node does not necessarily contain information for every property.
For example, a page node can never have values for the compute or literal properties. And while a value for the user data property is optional, a page node must always have values for the type and identifier properties.
The following table illustrates what properties may be in use for each node level.
yes — node can have that property
always — node always has that property
no — node cannot have that property