What is "Trigger start" service startup type?

What is the meaning of "Trigger start" when referring to Windows Services?

Triggers are events that allow the Service to be started as needed. Most Windows computers will have 20 or 30 Trigger start Services.

For example, when your antivirus updates your signatures, that might trigger a service to download the update. If you add or remove devices, that will trigger.

Another good example you probably used and didn't know is the Device Install Service. If you plug in a new keyboard, mouse, or USB device, the Device Install Service (DeviceInstall) will be triggered so that Windows can adapt to the hardware changes, often without any user intervention.

Source: Automatic and Manual Trigger Start Explained - MajorGeeks

Further Reading:

• Service Trigger Events - Win32 apps | Microsoft Docs

From Microsoft:

"A service can register to be started or stopped when a trigger event occurs... Examples of predefined trigger events include arrival of a device of a specified device interface class or availability of a particular firewall port."

Na example given at that URL is starting a service when a drawing tablet is plugged into a USB port. This avoids having the tablet service run needlessly.