Pulled this from SECNAV M-1850.1; dated Sept. 2019. Maybe this helps.
Board Composition
(1) The IPEB will normally be comprised of two Navy or Marine Corps Line Officers and one Navy Medical Officer O-4 or above. The Line Officers will be field grade or, for enlisted cases, may be a non-commissioned officer(s) at the E-9 level. At least two members of the board will be uniformed Service Members for all cases; a civilian equivalent to the officer member is authorized for three-member IPEBs. As needed to expedite case processing, PPEB is authorized to convene two-member IPEBs (one Line Officer and one Medical Officer). Two-member IPEBs will adhere to all requirements, specifically including the assignment of a third board member in cases with split opinions. At least one member of the board will be a uniformed Service Member for all cases.