In this context PSC means Person With Significant Control, normally 25% plus shareholders, but conceivably others with voting rights.
When the PSC regime came in during 2016 changes in PSC details only had to be notified annually, although people were encouraged to make event drive contemporaneous notifications to Companies House.
From June 2017 it is now a requirement that event drive contemporaneous filings are made – then likely triggers being changes in shareholdings or changes in shareholders details.
You can read more in our guidance on:
Filing a Confirmation Statement
Maintaining a PSC Register