Represents the "BitsPerFlag" entry of the type 6 shading
It is a required entry defined as part of the PDF 1.0
It contains an integer representing a PDF numeric object.
It should have a minimum value of 0 and a maximum of 3.
For definitive details see:.
The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 84,
The ISO PDF
Specification, ISO 32000-2:2017 PDF 2.0; Table: 83, page 244.