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You'll need to know what type of trains you're using on each segment of each trip, in order to determine whether you'll need reservations. That means you'll have to choose one departure time and buy reservations for any trains where they are required.
If France you'll need reservations for the TGV (high speed) trains, but not for the TER regional trains. Note that Passholder reservations in France are limited, and once the quota is sold out for a particular train you may not be able to buy them even if seats are available. Your only choice if that happens is to choose another train that does have Passholder reservations left, or forget the rail pass and just buy a ticket.
In Italy you will require reservations for Intercity and Freccia trains, but not Regionale trains. Note that reservations are specific to train, date and departure time and are only valid for the ONE train listed on the ticket. If you board the wrong train by mistake (even with a Railpass) you'll be subject to hefty fines which will be collected on the spot! Also note that AFAIK, Railpasses are NOT accepted on the Italo high speed trains.
I'm not sure about France, but in Italy Railpasses are generally not a cost effective option as tickets are relatively cheap. If you're willing to commit to a specific train, date and departure time on the express trains, significant savings are possible if you can get Super Economy tickets (which will include the reservations). Note that S.E. tickets are non-refundable and non-changeable once purchased, so choose carefully.
I'd suggest reading the Seat 61 website carefully, as a Railpass may not be the best option. You can check train details on the Bahn.de website.