It's definitely not just for u23s ptherwise why would our entire squad, barring the overseas players, be dual registered with dewsbury. Paul woods playing for Swinton tomorrow and he isn't u23. Afaik it is a two way arrangement too.

Championships Club. Dual Registered Players are subject to some restrictions on the competitions in which they are eligible to play, details may be found in the relevant competition rules. Clubs shall be entitled to enter into Dual Registration agreements as follows:
(a) All three parties (the Super League Club, the Championships Club and the Player) sign a Dual Registration form in such format as the RFL shall require from time to time;
(b) OnlyaPlayercurrentlyregisteredasaprofessionalplayerwithaSuperLeagueClub who is under 23 on 31 August in the year preceding the season in which the proposed dual registration will apply and who is outside the Super League club’s top 20 players (ranked by Salary Cap Value) for the Season in question can be dual registered;
(c) Only a Club in the Championships may “receive” a Player on dual registration;
(d) A“receiving”Clubislimitedtoatotaloffourloanedand/ordualregisteredplayersat any one time (in this respect a dual registration counts if it is current not just when it is being activated);
(e) A “receiving” Club is limited to a total of three loaned or dual registered players from any one Club at any particular time (in this respect a dual registration counts if it is current not just when it is being activated);
(f) Dual Registration agreements may be submitted at any time in the Season and a Player may be Dual Registered to more than one Club during a Season although he may not be subject to more than one Dual Registration agreement at any one time;
(g) ADualRegistrationagreementwillterminateonthedatestatedintheagreement unless it is extended or renewed by agreement of all three parties;
(h) ADualRegistrationagreementmayberescindedatanytimebythewrittenagreement of all three parties;
(i) The three parties to a Dual Registration may mutually agree to rescind the Dual Registration and enter into a Loan agreement subject to such Loan agreement extending to the end of the Season without any clauses allowing the Player to return to the Super League Club prior to the conclusion of the Championships Club’s final competitive match of the Season.
(j) It shall be for the two Clubs and Player to decide the details of the Dual Registration agreement, however the Player remains contracted to the Super League Club and as such the Super League Club will be entitled to decide for which team the Player shall be entitled to play in any particular round;
(k) Dual Registered Players are only eligible to play in the First Team of the Championships Clubs;
(l) The Salary Cap treatment of Dual Registered Players is set out in Section E1 and E2 of these Rules.

Those are the old dual reg rules. Obviously different rules apply for twinning agreements or Wood wouldn't be able to play for Swinton tomorrow.

They are the same rules per the current RFL website.
As for Wood playing for Swinton, I can only think that he was not dual registered and can go on loan for 28 days OR taking advantage of C1:2:21 (Loan Agreements) where the RFL are able to "use it's discretion" to allow special arrarangements.