Repeat just repeats a number of times, and you can only use a positive whole number.
For can use positive/negative values. For instance, you can
for x = -10 to 57
for x = 4 to 90003
The loop appears to only run forward though, so the end value has to be higher than the start value. Loopindex returns the current value, so if you start counting on 10 then the first loopindex is 10.
It might be handy to add a Step to the For loop, as in BASIC, so you could do like so:
for x = 1 to -10 step -1
that way it would count backwards. Or you could do
for x = 5 to 20 step 5
and it would count in increments of 5.