What I understand from reading the posts so far.According to Yankee, reloadable and disposable is a change of way to think about MLRS. Previously it's thought of as a different flavor of artillery. So the launch tubes are the guns and the rockets are ammo. When you need to fire you (re)load the tubes with rockets.
More recently these big 300mm and 370mm rockets are getting developed by people like CASIC, who naturally think of them as simplified missiles. To them it makes much more sense to have the rockets seal inside a capsule coming out of the factory as a single unit. Then the launch vehicles just have to worry about loading a whole module at a time, fire and get rid of the empty module, as you would say a pre-loaded ATGM. Its a different view on who the tube belongs to, the vehicle or the rocket. Being modular has a lot of advantages and is the way of the future, but one disadvantage is if the container is one use it's more wasteful.
So in this case there are a lot of 300mm rockets left over for PHL-03, those are intended to be reloaded into tubes and PLAGF don't want them to go to waste and would like PHL-16 to also be able to use them. Hence 4 cell 300mm rocket modules that allow reloading even though they're technically modular.
- 5th tube is loadable but isn't because of reusability concerns related with the existing PHL-03 rocket stock (reloading the module compromises the structural integrity?)
- When the 5th tube is fired, the module should be discarded as determined by the PLA (or manufacturer?).