In the Resource Sheet you will find the column Accrue At. With this you can determine when the costs will appear as actuals in your cost reports. For work resources for instance, the obligation to pay the expenses accrues when hours are worked. In this case, you would select the default option, which is Prorated.
Let's make it practical
Another situation is for instance if you have bought three PC’s that are used for a certain task. Once you start using these PC’s you incur the costs (possibly even sooner, at the moment of purchase). Regardless the duration of this task, if you incur the costs at the beginning you should select the option Start here.
The option End could be used, for instance for a task that has a fixed price or 'no cure no pay' assignment. The obligation to pay for this now starts to exist once you have verified the work was done correctly (there is a 'cure').
Do you remember?
Can you recall an example where you used this to determine when the costs need to appear as actual?
Leave it in the comments below.