how do you debug a material instance

yes that i know
i mean, how do you set a variable or parameter in a material so that you are able to read it from a blueprint
other than debug parameters i dont see a parameter that will accept an input value
e.g a component mask or a sphere mask will accept an input
a debug parameter will accept an input
you cant read the output from a sphere mask from a blueprint
seems you can only read the output from a debug parameter from a blueprint