For those who are still confused about the FM, CDF, and COAS Ranks and the Lifelong Immunity granted to FM
1. We currently have 2 Four-star appointments. CJCSC and COAS. CJCSC is senior to COAS on papers because COAS, CAM, CNS are members of the Joint Staff Committee under the leadership of CJCSC. However, CJCSC doesn't have any administrative and operational command over individual service branches. The role of CJCSC is the Principal Staff Officer which means this rank is the bridge between Civilian Government and Armed Forces, and as such the role entails providing Strategic and Military advice to Civilian Government and then relating the information back to Service Chiefs. CJCSC's role also involves in coordinating integrated logistics, finances, and information flow (strategic + military), and joint training b/w the services branches of the armed forces.
2. COAS, CAS, and CNS are the administrative and operational commanders of their respective service branches. So all matters pertaining to administration, logistics, and combat are under their control.
3. FM, in Pakistan's context is a ceremonial 5 Star appointment. Any kind of Marshal has the same set of power and authority as COAS, CAS, and CN.
The 27 Ammendent (as it stand before approval)
1. Seeks to first combine the offices of CJCSC and COAS into one office of CDF, and then expanding CDF's administrative and operational command to other service branches as well. (We'll know the nuances related to this administrative and operational command once after the ammendment's approval and subsequent restructuring of the armed forces).
2. The more interesting implication of this ammendent is the Life-long Immunity granted to FM. The basics of this immunity is the same as that granted to President and Governors in the constitution already i.e. they are not answerable to any court, can't have any criminal or legal proceedings against them, can have personal proceedings against them but with a notice period of 2 months, and can not be arrested. However, this immunity for Presidents and Governors is only applicable while they are in office, once they are out of office they lose this immunity. The 27 Ammendent seeks to provide this immunity to FM but for life.
This inherently implies that the rank of FM is for life, which simply means once a FM retires, they retire from administrative and operational command but continue to be treated as active officer and not SOD (Struck Off Duty) - they would continue to wear uniform and draw their salary (not pension), and other QoL benefits and now this life long constitutional immunity as well but won't have the administrative and operational power over armed forces.
The Loophole:
1. The loophole is the constitutional immunity. So for example, if a FM decides to invoke and implement Martial Law, they are not only protected against the breach of constitution but also low-key legally allowed
In all other cases, the FM will retire from administrative and operational command, and the government will find them another responsibility (related to coordination or security without any command) to justify their lifelong salary and privileges.
Hope that helps. I have kept it limited to information already available and not expanded into hypotheticals which will only become clear once the ammendent is approved (if approved).