"Do these 9 people represent an area? Or do they sit in the house 'at large', representing the whole Island?"
It's a little more nuanced than that - they represent values groups and communities that elected them, and are accountable to voters across the whole island.
Read more below...
The 9 island-wide MLAs are accountable to the people who elected them - so, they represent voters all across the island. But more than that, they'll likely represent a specific constituent group who voted for them.
To be elected from a list, MLAs will need to earn thousands of votes in their name alone, at least as many votes as are required to elect a District MLA now. The only difference from the current system in that those votes can come from all over the province.
This would allow, for example, the Acadian community to support an Acadian representative, the farming community to support a Farming representative, and so on - these are communities that share common values, but which cross district boundaries.
See also www.PRonPEI.vote/diversity to read more about how the list candidates can result in better representation of diverse island communities and constituencies, that are currently spread across many districts.
It is common in countries that have MMP systems, that the proportional-list MLAs also establish constituency offices, in places where that party is currently under-represented.
So for example, the NDP would have earned 3 seats if we'd had MMP in the 2015 election - the NDP MLAs may have established offices in Montague, Charlottetown and O'Leary, where the NDP vote was high, but where they did not win a seat.
There is an incentive for list MLAS to build strong connections in a geographic community - it increases the chance that they may be elected via the local district seats at the next election.