Development/Tutorials/Plasma5/Provides

The plasmoid metadata.desktop file can provide the key

 X-Plasma-Provides

It's a list, so one plasmoid can provide more than one feature

name	feature
org.kde.plasma.launchermenu	The plasmoid offers a "start menu" capability, a menu-based way to launch applications
org.kde.plasma.time	It's a clock in any form
org.kde.plasma.date	It's a calendar, or provides one
org.kde.plasma.multitasking	The plasmoid assists multitasking, such as a taskbar or a dock
org.kde.plasma.filemanager	Provides a filesystem view and some filemanager features
org.kde.plasma.trash	system trashcan access and control
org.kde.plasma.virtualdesktops	virtual desktop management tools, such as a pager or a button to trigger the show desktops effect
org.kde.plasma.activities	Activities management tools
org.kde.plasma.notifications	show notifications
org.kde.plasma.powermanagement	power management ifo, such as battery and brighness controls
org.kde.plasma.removabledevices	Removable devices info and access such as mount and unmount
org.kde.plasma.multimediacontrols	multimedia controls, such as info on what is now playing