MPEP 608.01(I) CFR 1.57
You can ONLY incoporate "essential" material from a US patent or app that does not itself incorporate by reference.
You can incorporate "non-essential" material from any publication.
"Essential" means that the information has to be there to comply with 112.
Here it is:
“Essential material” is defined >in 37 CFR 1.57(c)< as that which is necessary to (1) **>provide a written description of the claimed invention, and of the manner and process of making and using it, in such full, clear, concise, and exact terms as to enable any person skilled in the art to which it pertains, or with which it is most nearly connected, to make and use the same, and set forth the best mode contemplated
by the inventor of carrying out the invention as required by the first paragraph of 35 U.S.C. 112, (2) describe the claimed invention in terms that particularly
point out and distinctly claim the invention as required by the second paragraph of 35 U.S.C. 112, or (3) describe the structure, material, or acts that correspond
to a claimed means or step for performing a specified function as required by the sixth paragraph of 35 U.S.C. 112. In any application that is to issue as a U.S. patent, essential material may only be incorporated
by reference to a U.S. patent or patent application
publication. The practice of permitting incorporation by reference of material from unpublished
applications in which the issue fee was paid was discontinued by rule on October 21, 2004.
Other material (“nonessential subject matter”)<may be incorporated by reference to (1) patents or applications published by the United States or foreign countries or regional patent offices, (2) prior >and concurrently< filed, commonly owned U.S. applications,
or (3) non-patent publications **. Nonessential
PARTS, FORM, AND CONTENT OF APPLICATION 608.01(p)
600-95 Rev. 5, Aug. 2006
subject matter is subject matter referred to for purposes
of indicating the background of the invention or illustrating the state of the art.