The command field opens a transaction without walking through the SAP Easy Access menu. A transaction code names a business task: ME21N creates a purchase order, while MMBE displays stock. Use the identifier, not the screen title, when a colleague gives you a code.
From an active transaction, /n replaces the current task. /o opens the requested transaction in another session so you can compare it with the original. Unsaved work needs attention before you leave a form. An unfamiliar code should produce an error rather than open an unrelated screen. In these tasks, check the active transaction and the number of sessions before you continue.