After you file the trademark application, you may receive an Office Action from the USPTO non-finally rejecting your application based on likelihood of confusion, mere descriptiveness, primarily merely a surname, geographically descriptiveness, etc. There may also be less substantive issues raised, such a request for a disclaimer, a request for more information, or an issue being raised with the description of goods and services or the description of the mark. Often times research and legal argument is needed to overcome an office action. A letter of suspension typically issues when the Examining Attorney has no issues with your application except for the fact that someone filed a trademark application before you that might be confusable with your mark and the Examining Attorney wants to wait to see if that prior application registers before acting on your application. If, however, you have strong arguments that your application should register even if that prior application were to register, you usually can make them in advance of that registration.