The index in the selections list sorted as on BetFair. Regardless on their prices (Rank)
So, for example in Match odds soccer market the home team always has index 1, away team - 2, and the draw - 3.
For horse racing it's less useful because the index could change due to selection withdraw.
If you need to "save" selection for future use "remember" trigger action or sel_id variable (it's unique ID of the selection exchange-wide).