Answer for What’s the best format to use (csv/tab file) when loading source mailboxes into the migration window.

The batch wizard supports several different options for selecting user accounts.  Choosing the best option is a bit subjective, but may be helped with more detailed understanding of the options available.
TAB Import File
This simple feature allows for one to create a text file that lists all the source accounts to be selected.  The format of this file is very simple and is as follows:
The first line in the file must be:  DisplayName<TAB>DistinguishedName.  (the <TAB> is an actual ‘Tab’ character).
The rest of the lines in the file are the display name and AD distinuishedName values of the users to select.
Note that for each entry, validation is done by querying Active Directory and getting detail about each account.  If the list is very large (1000s of users), it can take considerable time to load the list – be patient.
 
The AD Browser utility
A very powerful feature is the AD browser utility.  This tool has several features to that can make finding and selecting accounts easy.
Containers
The first and most common way to select accounts is to simple navigate to the container or OU in the domain and select one or more accounts that are contained there.
Show object from sub-containers
This option, in the lower left of the browser, causes all mailboxes that can be found in the selected container and any sub-containers to be shown in a flat list.  If the root of the domain is selected, this will cause all mailboxes in the domain to be shown at once.
LDAP Filter
In conjunction with the above options, an LDAP filter can be applied to limit the objects that are returned.  Any AD attribute can be used as a filter, for example Department=*Sales* would show all mailboxes that have the word ‘Sales’ anywhere in the department field.  Note that LDAP filters are not case-sensitive.  For more detail, this link describes LDAP filters: link.
The LDAP filter only filters based on what would normally be shown based on the current container and the sub-containers option.  In order to have the filter apply to all users in the domain, be sure to select the root of the domain AND check the box for sub-containers.
Load list from clipboard
This feature, found on the left side of the browser, allows one to enter either very specific or non-specific details into a textbox from which an AD ANR (see this and this) search is performed.  This feature is most helpful when a list is already being maintained by another group or system.  For example, if the list of accounts to be migrated are in a SharePoint list, one could simply copy and paste either the email addresses or logon names of the accounts from the list and paste them in the ‘load list from clipboard’ dialog.  The tool will execute an ANR search for the objects.  If very unique values are used, a one-to-one matching will be resulted.
However, and sometimes more convenient, if a more ambiguous value is used, all accounts matching that value can be returned.  For example, if all accounts with ‘rich’ in the name needed to be selected, simply entering ‘rich’ would work.  ‘Richard’, ‘Richardson’, ‘Dunrich’, and ‘Goodrich’ will all be returned.  This can be especially helpful in finding an account for which only some detail is known.
Load Selections
This feature allows one to load a previously saved user list file.  The page on which database assignments are made has an option to save the selections to a file.  The resulting file is a binary file that contains the selected source accounts, the matching target accounts, and any database assignments that were made.
Priasoft recommends using this feature as it does not go thru the validation steps that the other options have.  This can be valuable to avoid delays in the start of a batch.  In addition, because all the details are recorded, an administrator can hand the work off to someone of no experience to load the list and the migration will work on the users as the admin designed.
Furthermore, selecting accounts can be a time consuming endeavor.  It is best to do the selection work a few days before a production migration and save to a user list file.  There can be cases where questions are raised about who to migrate, or cannot be found, and the like that if faced with such on migration day, can delay the start until such things are sorted out.