Event Status Codes are used to designate whether an event is open or closed to online registration. When adding or editing an event in the AMS, the Status field is filled by selecting from a drop down menu, as seen in the next screen shot. 
The Event Status Codes Dictionary is a flat table where the choices for the drop down menu above are defined. You can have as many MasterCodes as you need, and the most common are seen in the next screen shot for the sample GroupType Event Status Code (EVSTATUS).
Two statuses that must be available are Registration Closed and Registration Open. The latter status is required for meetings that have online registration in order to activate registration on your website.