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Your SSO service will have a unique user record for each person using it within your company. When accessing a Box application through this SSO service, if we’re creating a Box user for each SSO user, then we need to create an association between the SSO user and Box user records. When a user logs in to Box through the SSO service we will first search for the user by the association. If a Box user record is found we can begin making calls as that user to Box APIs. If there is no Box user found will then create a new Box user with the association to the unique SSO user account. Exploring the top level of a Box user object we can see the available options for adding the unique identifier from the SSO service user object into the Box user object to bind the two together. 
{
  "address": "900 Jefferson Ave, Redwood City, CA 94063",
  "avatar_url": "https://www.box.com/api/avatar/large/181216415",
  "can_see_managed_users": true,
  "created_at": "2012-12-12T10:53:43-08:00",
  "enterprise": { .. },
  "external_app_user_id": "my-user-1234",
  "hostname": "https://example.app.box.com/",
  "id": 11446498,
  "is_exempt_from_device_limits": true,
  "is_exempt_from_login_verification": true,
  "is_external_collab_restricted": true,
  "is_platform_access_only": true,
  "is_sync_enabled": true,
  "job_title": "CEO",
  "language": "en",
  "login": "[email protected]",
  "max_upload_size": 2147483648,
  "modified_at": "2012-12-12T10:53:43-08:00",
  "my_tags": [ .. ],
  "name": "Aaron Levie",
  "notification_email": { ... },
  "phone": 6509241374,
  "role": "admin",
  "space_amount": 11345156112,
  "space_used": 1237009912,
  "status": "active",
  "timezone": "Africa/Bujumbura",
  "tracking_codes": [{ .. }],
  "type": "user"
}
There are two recommended methods for creating a binding between a unique user within the SSO service and a Box user, placing the unique SSO user ID within the Box user external_app_user_id field, or using the unique SSO email address as the login email for the new user. The external_app_user_id field was designed to hold a string identifier to associate a Box user record with an external service, such as an SSO provider user record.
You can retrieve app users for a specific application only if such app users were created by this application. If you use one application to search for users created by a different one, no data will be returned.
Using the external_app_user_id field for associating the unique SSO user account with a Box user account is the preferred method of connecting the two accounts over email, for a number of reasons:
  • Email association is only viable for managed users. App users are automatically assigned an email address by Box, meaning that you cannot assign the login to be the email from the SSO service.
  • Emails have to be unique in Box. This means that if your SSO service user signed up for Box using the same email address, which is not within your Box enterprise, then you will not be able to create a user with that email and won’t be able to connect to that existing user.
  • The external_app_user_id field was designed for this purpose.

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Using login (alternative method)

Using the login field of a user object to create an account association is viable under a few conditions:
  • Only the managed users type is being used, not app users.
  • All email addresses and Box account creation requests are managed by your enterprise, meaning that users cannot independently create Box accounts with those email addresses.
Email addresses used for users in Box, under the login field, must be unique. Making a request to create a user with an email that already exists for another account will result in a 409 Conflict error, stating that user_login_already_used