There are 9 warp rings available, numbered 0-8. Each ring has a configurable number of entries in it. As an example, in the first quest, ring 0 is used for the 3-Stair Warps, so there are 4 entries in it (one for each location Link may use for a warp). The way those work is that if you are at location N, the stairs take you to location N+1, N+2, or N+3, depending on which path you choose. The number will wrap around if it is greater than the count given in the Warp Ring dialog, so that the list is circular (hence the term warp ring).
By default and tradition, warp ring 8 is used for the whistle. Its warp points are only active if Link has collected the Triforce piece corresponding to the warp ring entry number. Note that the position of the level entrances has nothing to do with the screen that is warped to by collecting the corresponding Triforce; it is still set by the warp ring dialog.