There is no shortage of issues arising from US restriction rules are regulations. One feature that seems strange is the fact that same application filed in the US can have two very different sets of restriction rules applied based merely on the formality of how the application is filed. This is the case where file a national application from PCT uses the unity of invention approach, whereas if the application is filed as a continuation (e.g., the by-pass route) of the PCT application, then regular US restriction rules apply (independent and distinct).
Let’s say you file your PCT, get a unity of invention restriction, make an election, and prosecute the claims to issuance while filing a divisional application for the non-elected invention. What restriction rules apply to the divisional?
Logic might lead one to think that the USPTO should keep applying the PCT unity of invention rules, but like much of what the PTO does, logic will only lead you astray. Under MPEP 823, the independent and distinct analysis applies. So, you could have the very real situation of a completely new set of restrictions on the divisional application as compared to its parent national phase application. From MPEP 823:
Unity of Invention Under the Patent Cooperation Treaty
The analysis used to determine whether the Office may require restriction differs in national stage applications submitted under 35 U.S.C. 371 (unity of invention analysis) as compared to national applications filed under 35 U.S.C. 111(a) (independent and distinct analysis). See MPEP Chapter 1800, in particular MPEP § 1850, § 1875, and § 1893.03(d), for a detailed discussion of unity of invention under the Patent Cooperation Treaty (PCT). However, the guidance set forth in this chapter with regard to other substantive and procedural matters (e.g., double patenting rejections (MPEP § 804), election and reply by applicant (MPEP § 818), and rejoinder of nonelected inventions (MPEP § 821.04) generally applies to national stage applications submitted under 35 U.S.C. 371.
But do not despair. If this situation occurs remember that one tool available to push back is the notion that all restrictions require a serious search burden. An applicant faced with a troubling restriction in the facts above may consider explaining how the search performed in the parent application might apply to some of the allegedly restricted claims, thus reducing the search burden. Or, the applicant might be able to traverse the restriction in other ways.
If a difference in which restriction rules apply to an application might affect the outcome in a substantial way, an applicant may consider purposely selecting the nation phase or by-pass route in the hopes of more desirable restriction rules. Which way that cuts is difficult to say in the abstract, but may become more clear for particular situations. However, the unpredictability of restrictions could likely override the utility of any such strategy.