An Internal Write Parameter provides a connector to which you can wire on a function block diagram so that you can write a particular parameter. The parameter connector is not seen outside of the block or module by default. You can use the Show Parameter function to make an Internal Read parameter appear as an output connector on a module block or composite.
Make sure this parameter does not reference an external reference parameter from within this same module. (You can configure this, but it will cause problems at run time.)