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.virtualdesktop	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