After being assigned to a certain cadre, IAS officers usually work in the same cadre throughout their service, unless they go for central government deputation. In some rare cases, they may also be allowed to change their cadre.
DoPT allocates cadres to candidates selected for All-India Services
The Department of Personnel and Training of the Government of India allocates cadres to candidates. Notably, the assignment of cadre depends on the candidate's rank, preference, and vacancies.
The All-India Services' cadre allocation policy was revised in 2017. At the time, the existing 26 cadres were divided into five zones. Under the new policy, candidates are required to choose cadres from these zones.
Zones #2: About Zone-IV and Zone-V under the new policy
Coming to Zone-IV of the new cadre allocation policy, this zone consists of six cadres, including West Bengal, Sikkim, Assam-Meghalaya, Manipur, Tripura, and Nagaland. On the other hand, five cadres constitute Zone-V, which includes Andhra Pradesh, Karnataka, Kerala, Tamil Nadu, and Telangana.
Under the new policy, candidates have to indicate their first-choice by selecting a cadre/state from one zone.
Their second cadre/state choice should be from a different zone, third from another zone, and so on. After one preference from each zone is indicated, they should give their second cadre choice from each zone and continue in the same manner for indicating preferences in descending order.