A user assigned to the primary domain can post content to any domain by changing URL.

For the setting to work, the user must have the 'View domain publishing' permission.

See 4.2.2 of the README, pasted below.

4.2.2   Content Editing Forms

Defines how to present the forms for node creation and editing to users
who do not have permission to 'set domain access' but need some control
over where their content is published.

Users with the 'view domain publishing' permission will be subject to the
rules defined below.

  -- Pass the default form values as hidden fields
  The default option.  It will assign all content to the root domain and to
  the domain from which the form is entered.

  -- Take user to the default domain
  Before being presented the editing form, users will be taken to the root
  domain.  If the node is not visible on the root domain, the user may not be
  able to edit the node.

  -- Take user to their assigned domain
  Before being presented the editing form, users will be taken to the
  first domain assigned to their user account.  This function is most useful
  when you users are only allowed to enter content from a single domain.

  Note that for users who have more than one assigned domain, this option
  will take them to the first match and the user will not be allowed to
  change the domain affiliation.

  -- Show user their publishing options
  The node editing form is shown normally, and the user is presented a
  list of checkboxes.  These options represent the affilaite domains that
  the user is allowed to publish content to, according to the domains
  assigned to their user account.

  Note that if this option is selected, users with the 'view domain publishing'
  permission will also be shown a list of affilates to which the node is
  assigned.  This list shows only the affiliates that the user cannot edit.

  Warning: If this option is selected and the user has no domain publishing
  options, the user will not be allowed to post or edit!

Note also that the user is not given the ability to promote content to
'all affiliates'.  Users who need this ability should be given the 'set domain
access' permission instead.

OK...I created a main domain and a sub-domain. Created a user for the sub-domain and have set the domain access settings and everything seems to be working fine except one. When a subdomain user goes to create content the checkbox next to "Send to all affiliates" is selected by default. So when the sub-domain user posts something, it automatically appears on the main domain and I as an administrator have to go in and uncheck "Send to all affiliates" manually.

So the question is, where do I make the change so that the checkbox next to "Send to all affiliates" on the content edit page is "UNCHECKED" by default.

Thanks in advance for the help.

See sections 4.2.1 and 4.8.2 of README.txt