This is the organisation that a licenced doctor has a professional, educational or employment connection with that provides them with support for revalidation. 

In London and the South East (LaSE), there are four Designated Bodies corresponding to the three London Local Offices and KSS.

Your welcome letter from HEE should help you to determine which region you should be connected to.

For a full list of regions and responsible officers please see the FAQ "Who is my responsible Officer".