Selection Parameter - Index

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).

Do you mean 1 for the favourite, etc.? It's "Selection's Rank" option or sel_order trigger variable.
If you need the place the selection got after the market settlement then sel_place (the market have to be settled of course).

Yes, then it's "Selection's Rank" or sel_order trigger variable. Available all the time, not in-play only.
So it's selection's index in the list sorted by back price, then by lay price, then by last traded price:

Back   Lay   LTP   Rank
1.1    1.2   1.3   1
2.1    2.2   2.2   2
2.1    2.3   2.1   3
2.1    2.3   2.3   4

Which may not be in price order of course?

I'm sure it's the price. Why not?

Unfortunately there is no way to "see the race" with the software. And BetFair (and nobody) doesn't provide such information as number in realtime.