A Practising Certificate is required when a member engages in practice.  Practice is the provision of services including accounting and related services for which the relevant skills have been acquired by a member by reason of his or her training and qualification, to persons other than his or her employer.

A member engages in practice when he or she provides, or holds him or herself out to provide, such services to the public either personally or through a firm.

Examples of some of the more common reasons for requiring a Practising Certificate include:

• Becoming a principal in an accountancy firm;
• Independently offering accountancy or related services to the public;
• Being an insolvency permit licence holder; or
• An employee Responsible Individual under the Audit Regulations.

Further information can be found here