The second last argument tells the "period" of bars within which to find the bar of the lowest value (as selected by the third argument), and the last argument tells the bar index of the latest bar in that period.

Thus, "iLowest(...,MODE_LOW,1,1)" points at which bar within a period of 1 bar, ending at bar 1, has the lowest price (of that period).
And "iLowest(...,MODE_LOW,1,0)" points at which bar within a period of 1 bar, ending at bar 0, has the lowest price (of that period).

Obviously, using iLowest for a period of 1 bar is a bit of a waste. That is, "iLow(NULL,0,Lowest(NULL,0,MODE_LOW,1,1))" is exactly the same as "iLow(NULL,0,1)" except that it uses some few more cycles.

Also, as you know, "iLow(NULL,0,0)" is the *currently* lowest of the bar currently being developed, and this might change as the bar develops.