Substitute items usage

Hi All

Please help me understand under which circumstances(SOH at the warehouse) the substitute item will be transferred fro warehouse to the store and not the main item.

Also please qoute how is the fill priority be used which deciding the transfer quantities.

Questions

1) Will fill priority only define the order in which the the main and the subsitute items will be replenished?

2) What will happen if the main item does not have any SOH in the warehouse?

3) In case the availablity in the warehouse and the store need of all the items(main and substitute) on replenishment is same, will all the items be transferred to store or only the main item?