1) AP users configured from Default partition are first created on Default partition and then bound to a Admin partition.
But for the other case, you need to first switch to the admin partition and create the user in that partition itself.
2) You can only login to ADC through NSIP and SNIP of default partition using the default partition user admin . Whereas you can only login to ADC through SNIP of Admin partition using admin partition created user.
3) You can bind user defined command policies for default partition user. But for admin partition specific user can have only 4 pre-defined command policies to choose from.