The continuation period (sometimes known as the writing-up period) is to allow for contingencies; you should be planning to submit your thesis at the end of the normal period of enrolment.
Your continuation period begins at the end of your normal period of enrolment. During this time you will retain access to the Library and IT facilities. You may only access departmental facilities with the written permission of the department's Director of Postgraduate Researchers. Postgraduate researchers entering a continuation period will be liable to pay a continuation fee.
If your studies are funded by a grant-making body (e.g. WRoCAH), they may have a particular timeframe for you to submit your thesis. Please check with your supervisor and PGR Administration.