There is an option in “Edit server -> Notification settings” as “Send notification when alert-able” wherein you can give integer values like 2, 3 etc, which means that MONyog will send you alerts only when a counter is in alert-able state for this (2,3 etc.) many data collection interval times consecutively, this setting is server level, meaning it will be applicable for all the counters and monitors.
The “Override send notification when alertable?” is a monitor level setting, and is used to override the value that you set for in the “Send notification when alert-able” option. So, if you have set up a value as 3 for “Send notification when alert-able”, then you can override this using the option “Override send notification when alertable?” for any monitor, if you want to get an alert for a lesser number of intervals for that monitor.
The value for this totally depends on your need, you can set a value 1, if you want to have an alert the first time counter gets into a critical condition or any higher value if you think the counter is not critical enough to look into if it is alert-able only for one data collection interval.