which rack

Which rack are you supposed to use in the oven? For baking cookies, cake, bread…pizza, chicken, fish etc. Does it matter or should all racks --lowest/middle ones/highest— be the same once the oven is heated?

Always Wondered, Never Knew
Thanks

Very good Qs. Also let me know for baking of cakes i`ve heard that for the 1st 15 min u turn on the lower electric rod only and after that both upper and lower rods.....is it true........to facilitate rising of the cake initially in 1st 15 min

*thanx *