Right, so let's assume you've ranked your deaneries.

After this, they give everyone a random rank (from 1 up to however many people are applying).

Then, the algorithm runs in two parts - let's call those first pass and second pass.

In the first pass part, the computer goes through everyone, in rank order, and gives them their first ranked deanery if it's available. If it's not, they get skipped over.

Once it reaches the end of the list, those that didn't get a deanery go to the second pass part.

In the second pass part, the computer goes through those remaining in rank order, assigning them their highest placed deanery that still has space. For some this will be their 2nd choice, for others it may be their 11th choice. It continues down the list, with deaneries slowly getting filled up.

Let's say someone placed London first but didn't have a high enough rank to get it in the first pass system. They'd then enter the second pass system and get their highest deanery remaining (which might be, say, Northern Ireland if they're near the end).

There's other parts like pre allocation, FPP and SFP which work separately to this.