Mainland production numbers go to new islands (Settlers of Catan app)

Question

When a player reaches an island by ship, the hex they "land on" gets a production number. However, if the computer has run out of new production numbers to assign, the production number for the newly-reached island hex is taken from a hex in the mainland (i.e now a hex in the mainland has no production number).

When this happens, the computer lets you pick which number from the mainland you want to take to the island. I would like to know how the game decides which choices to give you. It doesn't always let you pick from all of the hexes you currently occupy on the mainland.

Answer 1

The rules are given at Catan Maps (and I believe somewhere in app, but I don't have it installed at the moment):

After setting up the board as shown, set aside the following four number tokens, shuffled in a face-down pile: 3, 5, 8, 9, 10

[...]

As soon as a player’s ship reaches the intersection of a terrain hex belonging to one of the small islands, a number token is assigned to this hex. The first 5 number tokens are taken randomly from the face down supply.

Once the supply is used up, the player whose turn it is must remove a number token from the main island. At first, only number tokens are available that obey the following 3 rules:

1. The number token must belong to a terrain hex adjacent to which the active player owns a settlement or city.
2. Each settlement or city on the main island must keep at least one terrain hex with a number token.
3. On a small island, the numbers 6 and 8 must not be next to each other.

If it is not possible to comply with all three rules, you may break the rules in consecutive order from 1 to 3.

The page at Catan Maps also notes that this scenario was included in the first edition of Seafarers, but was removed because it requires so many different sets.