New rules come into force from April which mean that agencies have to supply HMRC with information on all the workers they place with clients even if the agency doesn't pay the worker (which is the case when you work through an umbrella company, work using the CIS scheme or have your own PSC).
You may find that your agency asks you to confirm certain information. If you are an umbrella employee or CIS worker this is likely to be:
We hold all this information for you which you can access via the portal.
If you have your own PSC you may be asked to confirm your personal details as above but also details relating to your PSC such as the company number.
Some agencies already hold the information they need but may still ask you to confirm it is right since there are fines for supplying incorrect information to HMRC.