Part of macOS
useractivitydHandoff
○ Leave it
Passes what you’re doing between your Apple devices.
What it is
Runs Handoff and Universal Clipboard.
If you stop it
macOS starts it again straight away. Leave it.
The verdict
○ Leave it. Part of macOS, or something that starts again by itself. You never need to close these.
In Activity Monitor
useractivitydUASysAgentcompaniondnearbyd
From Procession’s guide, in the words the app shows.