According to the Citrix documentation1, the triggerMonitor parameter specifies the conditions under which the service monitor is invoked. The possible values are:
ALWAYS: The service monitor is always invoked.
MEPDOWN: The service monitor is invoked only when Metric Exchange Protocol (MEP) has marked the service as DOWN for any reason.
MEPSUSPECT: The service monitor is invoked only when MEP has marked the service as SUSPECT.
NONE: The service monitor is never invoked.
Therefore, the command set gslb site SiteB -triggerMonitor MEPDOWN will set the triggerMonitor value to MEPDOWN for SiteB, which means the service monitor is invoked only when MEP has marked the service as DOWN for any reason.
1: https://docs.citrix.com/en-us/citrix-adc/13/gslb/configure-gslb-site.html