purchase release and autocreate requisition

Hi,
I wanted to know what is the difference between purchase release and autocreate requisition? Say you have a sales order with an order with line having a base ato model.
my understanding is that for external drop ship (source type external) you run the purchase release program which will create a purchase requisition.
and when  you run the autocreate requisition(if the source type internal), say, for a config star item then also it is creating a purchase requisition.

so can we use any one of them ? can anyone tell me the difference?
Thanks.