The PORAC Retiree Medical Trust is a funding vehicle in which to deposit money while you are working, to pay for medical expenses after you retire, pursuant to the rules of the Plan. Any PORAC member Association may choose, as a bargaining unit (through collective bargaining), to join the Trust. (You cannot choose to join as an individual; it must be through bargaining by your Association.)
Note regarding new hires: An Association may bargain that only new hires will participate in the Trust, e.g., every employee hired after January 1, 2017. That is, although there can be no individual employee choice to join the Trust, an Association may bargain that a certain defined class of employees will participate in the RMT. That may help in situations where your employer is cutting off retiree health benefits only for new hires.