The supply alerts trigger can be determined by the threshold set by default or can be customized. The threshold defined in the Supplies tab  of each printer, will affect the trigger of the alert on the supply alert report, while the Account threshold will be used to trigger the alert in the supply alerts dashboard

The supply alets must be catched on the fly as once the cartridge is replaced the alert will not longer be available. The recomendation is to suscribe to supply notifications to get one alert per supply. Another option is to schedule emails using supply alerts report often, with a frequency of for example, once a day

The severity of a Supply Alert in the Dashboard refers to the Filter set in the same Dashboard:

• "Warning" for low on Ink. If you need to see the alert after the specific Threshold (20% by default) has been reached, select Warning only as default.
• "Error" for cartridge depleted. If you leave Error as default you will see the alert when cartridge out of ink in recevied by Partner Link server 

You could decide to get an alert for the same cartridge twice by selecting both settings. The intent is you are aware, to prepare a new supply for delivering to the customer. 

Same behaviour applies to the case when you want to receive a Notification email (Email alerts : On or Off) . 

Note: Be aware the Ink Level seen in the Dashboard is not real time but updated periodically (aprox every 3 hours) with the latest Level of the printer supply as received by Partner Link server. It may not be not anymore the current supply in the printer, and not anymore the level of the cartridge when the alert was generated (according to the Threshold).