How Regionalism Will Work
Simple Terms: More regionalism makes you more likely to win some seats but less likely to win a lot of seats.
Every party will have a regionalism % between 0% and 100%. This % will determine how many constituencies that the party is competitive in. A 0% regional party is competitive in all constituencies. A 50% regional party is competitive in half of all constituencies. A 90% regional party is competitive in just 10% of constituencies. A 100% regional party is competitive for just one seat. A party starts with either 0% or 100% regionalism and can alter regionalism by up to 10%/election, meaning that regionalism builds and fades slowly.
Your party's PP is divided over the constituencies your party is competitive in. IE: A party with 100 PP overall that is competitive in 5 constituencies has 20 PP per constituency, while a party with 100PP overall that is competitive in 50 constituencies gets 2 PP per constituency. The party with the most PP wins the constituency.
Or, to put it in simpler terms, more regionalism concentrates your PP, making it more likely you will win seats but decreasing the number of seats you can win.
If a party has at least 10% regionalism, they'll need to pick which regions they're competitive in. IE: A party might be competitive in Sardinia and Piedmont. This determines where the seats you're trying to take are. The higher your regionalism, the fewer places you're competitive in.
In the case of regional overlap (two or more parties competitive in the same region) and an uneven seat split, the seats go to the party with the highest number of PP first assuming noncompetition. IE, suppose:
In Region X there are 10 seats
Party A has 5 PP/seat over 5 seats
Party B has 4 PP/seat over 4 seats
Party C has 3 PP/seat over 10 seats
Then 5 seats go to Party A, 4 to Party B, and 1 to Party C. In other words, the ambiguity breaks against the party or parties with the lowest PP/seat.
Ties will be broken by even splits and coin tosses.
If there are some seats no party competes for, then they will become independents.