Ah, now that's confused me happy. I don't think the Scottish Parliament elections do use ranking, so first/ second choice etc? I thought that was just for the council elections.
As I understand it everyone gets 2 ballot papers - one for the constituency and one for the region. You put a single X on both... for an individual candidate for the constituency (which is counted up, and the one with most votes wins, so FPTP) and for a party for the region. Each party has ranked their candidates already in their "list" so you don't get to chose which one you want as a voter. If a party wins a list seat, the first person on the list gets it, if 2 seats the first 2 and so on.
The seats are allocated by counting the number of votes for each party, but then dividing that number by the number of constituency seats that party had already won within the region (plus 1). So if every snp voter in a region gave their second vote to SNP and they got, say 100,000 votes, but had already won 9 constituencies, the list vote would be 100,000/ 10, so 10,000. However, if 40,000 people voted Tory, labour, Alba or whatever, and they had won no constituencies, their list vote would initially divided by 1 (40,000), so they would get 1 MSP. Then it would be divided by 2 (20,000) which is still higher than the SNP, so their second list candidate would be selected. Then 3 (13,333) which is still higher, so their 3rd list candidate would be selected, and only at the 4th list seat would the SNP get a go. Obviously it's not as simple as that, as there are more than 2 parties running, so it's possible that the SNP wouldn't get any list seats at all if they do well in constituencies, with the parties with no constituencies getting 1 or 2 list seats each out of the 7(?) available in each region.
So if each region has 1 or 2 alba seats, that could potentially be more pro independence MSPs in parliament than if people give both votes to the SNP. It's a tricky balance to play though, as if the SNP doesn't do so well in constituencies they could well pick up a few list seats, but are less likely to do that the more votes go to alba! It is a totally legitimate strategy by Alba, but it does run the risk of disenfranchising unionist voters who may not be as well represented proportionally in parliament as their numbers amongst the electorate. Though if the SNP don't do as well in constituencies as predicted (or in regions where the constituency winners are a bit more varied), it may indeed stop there being an pro independence majority if the list votes don't all go to the SNP...
Utterly baffling as far as I'm concerned, and my brain can't cope with the various implications, but quite interesting anyway!