This option was added with Software Delivery Solution 6.1 Hotfix 002 and can be a bit confusing. There are four options in the drop down list
The first 3 options should be apparent;
This last one is confusing because the traditional meaning of "prestaging" is analogous to preloading before use. Technically this is correct, but there is a difference in the actual behavior. This option is used by choosing a collection of computers to which the package in question needs to be distributed. This option then automatically chooses the appropriate package servers. The "prestaging" will allow the selection of additional sites that contain computers which may need the package in the future. This is what is meant by prestaging.
To try and illustrate I will refer to the following image:
A collection is chosen that contains client computers from both sites A and B but none from C. The automatic selection will choose the package servers from these two sites, A and B, but not from Site C. If there may be a need for the package in question to be sent to client computers in Site C then that site can be chosen with the Advanced tab and the package will also be sent to the package server in Site C. This presumes that the package server is already in place
This option does not preload a package server before its actual deployment into the network infrastructure.