When you initially send out your applications, you will send them to a lockbox. That's the place where they check to make sure that you sent in the correct fee and the correct applications and that they're all signed. After they have checked everything out, the lockbox will transfer your application to a Service Center for processing. Once your applications get to the Service Center, they will mail you two Receipt Letters that state your receipt numbers.
The form G-1145 just emails and texts the receipt numbers to you, which you will get like 2-3 days before the Receipt Letters arrive in the mail from the Service Center. Like I said, you won't receive any information that you won't get by mail, but it's still exciting seeing the text and emails from USCIS to know that your process has started.
Here is the form. Place it on top of the other applications.
http://www.uscis.gov/portal/site/uscis/menuitem.5af9bb95919f35e66f614176543f6d1a/?vgnextoid=d9056d4e88ac3210VgnVCM100000b92ca60aRCRD&vgnextchannel=db029c7755cb9010VgnVCM10000045f3d6a1RCRD