All regional rural banks (RRBs) are not part of the participating banks for the IBPS RRB PO Exam. The Institute of Banking Personnel Selection (IBPS) conducts the IBPS RRB PO Exam to recruit Probationary Officers (PO) for various regional rural banks across India.

Participating Banks in IBPS RRB PO Exam:
The participating banks for the IBPS RRB PO Exam include:
1. Regional Rural Banks (RRBs) - These are banks that are specifically established in rural areas to cater to the banking needs of the rural population.
2. Public Sector Banks (PSBs) - These are banks that are owned and operated by the government.
3. Cooperative Banks - These are banks that are owned and operated by a group of individuals or organizations.
4. Financial Institutions - These are institutions that provide financial services, such as loans and investments.

Not all RRBs are participating banks:
While all regional rural banks are eligible to participate in the IBPS RRB PO Exam, not all of them choose to do so. Some RRBs may choose not to participate in the exam due to various reasons, such as their recruitment needs or their own internal recruitment processes.

Importance of participating banks:
The participating banks play a crucial role in the IBPS RRB PO Exam as they provide the vacancies for which the candidates are recruited. These banks provide the pool of vacancies that are filled through the exam, and the successful candidates are offered the position of Probationary Officers in these banks.

Selection process:
The selection process for the IBPS RRB PO Exam includes a preliminary examination, a main examination, and an interview. Candidates who qualify in the preliminary examination are eligible to appear for the main examination. Those who clear the main examination are then called for an interview. The final selection is based on the candidate's performance in the main examination and interview, subject to meeting the eligibility criteria of the participating banks.

In conclusion, while all regional rural banks are eligible to participate in the IBPS RRB PO Exam, not all of them choose to do so. The participating banks provide the vacancies for which the candidates are recruited, and the final selection is based on the candidate's performance in the main examination and interview.