(Required) An array of four numbers in the form coordinate system, giving the coordinates of the left, bottom, right, and top edges, respectively, of the form XObject’s bounding box. These boundaries are used to clip the form XObject and to determine its size for caching.

(Optional) A code identifying the type of form XObject that this dictionary describes. The only valid value defined at the time of publication is 1. Default value: 1.

(Optional; PDF 1.4) A group attributes dictionary indicating that the contents of the form XObject are to be treated as a group and specifying the attributes of that group (see Section 4.9.2, “Group XObjects”). Note: If a Ref entry (see below) is present, the group attributes also apply to the external page imported by that entry, which allows such an imported page to be treated as a group without further modification.

(Optional) An array of six numbers specifying the form matrix, which maps form space into user space. Default value: the identity matrix [1 0 0 1 0 0].

(Optional but strongly recommended; PDF 1.2) A dictionary specifying any resources (such as fonts and images) required by the form XObject.

(Required) The type of XObject that this dictionary describes; must be Form for a form XObject.

(Optional) The type of PDF object that this dictionary describes; if present, must be XObject for a form XObject.