File Allocation on z/OS Systems.
On a z/OS system all disks are available to all users with different users being able to access any disk concurrently with
other users, subject to security system rules.
File allocation is performed at the data set level and hence a specific allocation is required for each data set, either
dynamically via a command, such as TSO ALLOCATE, or statically via a JCL DD statement.
File Allocation on z/VM Systems.
In the case of z/VM CMS, assignment occurs at the mini-disks level, and these are assigned via the the Users Directory.
This means that File Allocation as such is only required if the file to be used is not held on the default set of mini-disks.