No extra from what I understand, have never really looked into all details. SO: if is a cheaper pass-which means the fee to pay in case you need to give back is also relatively smaller, it may not even be worth it at all. Since the central organisation that came in force with the new style mobile passes they are pretty good at imitating RYAN&the like in ways to shake more coins out of you. Those with beezneez expertise can probably even think of more ways on how to do that

(RYAN: not pay for chosen seat-OH! you as single traveller cannot sit with your family!

The plus is simply an insurance policy on your refund level. It simply underwrites any loss you may incur if you have to return your pass for a refund (i.e. the 15%). It also allows you to exchange your pass without the 15 euro charge.

One thing to bear in mind that, as with most insurance premiums, you do not get the PLUS fee returned, so the cost of PLUS has to be factored against the 15% loss if you request a refund.
https://www.interrail.eu/en/interrail-passes/ordering-info/secure-your-pass

I am not sure whether PLUS is an option with promotional passes that are often not refundable, but (if included) they and the more expensive passes are probably the only ones worth considering covering with PLUS. 25 euro extra for PLUS would only give a break even return on standard returned passes costing nearly 200 euro and one costing 500 euro would “save” a net deduction of 50 euro. If you use even one day of your pass it is no longer refundable.

I wonder myself, as the exchange policy states: “If you purchased Plus, together with your Pass, you could exchange your Pass for free. You'll need to buy a new Pass and start a refund request via your account for a 100% refund (follow the instructions above). However, costs for Plus are non-refundable.”

Do I understand this correctly to mean that if you exchange a pass with Plus, you in fact pay €24 exchange fee (the plus fee)?

In the following theoretical examples, is my understanding correct?

• I buy a pass for €400+€23 for Plus. I exchange it for another pass costing €300. I get refunded €100. Netto, I end up paying €323 (or €346 if I also add Plus to the new pass?)
• I buy a pass for €400 and no plus. I exchange it for another pass costing €300. I get refunded €85. Netto, I paid €315 (or €338 if I add Plus to the new pass)

If this is correct, for exchanges adding plus actually makes the passes less flexible. It would in my opinion make sense that plus is refundible for exchanges (particularly if exchanging against another pass with Plus), but the wording of the exchange terms seems to suggest that it isn’t the case.

Overall, I’ve always avoided Plus as the fact that you don’t get the plus costs mean the difference with an 85% refund is pretty small - and for some specific passes it is certainly not worth it (unless you’re certain you will cancel - in which case, don’t buy the pass). For example: the €167 4 day youth pass. If cancelled with plus, I don’t get refunded the €23 plus fee. If cancelled without Plus, I don’t get refunded 15% = €25.05. If I end up using it, with Plus, I’ve paid €23 too much.

Honestly, I think the flexibility of Interrail is something that makes it great, so I’m not sure why these kind of confusing upgrades are necessary.

Your understanding is the same as mine - you do not get the Plus fee back in any case, so on the shorter passes up to about 180 euro you actually lose out on refunds (except possibly non-refundable promotional passes, but that needs confirming). After  that the saving is obviously 15 euro per extra 100.

Exchanges are always a loss because the saving on any “free” exchange is 15 euro against a cost of 23 euro plus another 23 euro on the replacement pass (if you take it).