This gets a little confusing. White ( in the computer) can be 0 % black and is called different names. This is a computer white, a knock out white and also a CMYK white. The only thing it is not, is a spot color white. Spot color whites are used when you actually need to print a white ink. In order for it to print as a separation. It needs to be a custom spot color.
A process white has 0% in each of the CMYK and therefore, knocks out of any color it is on top of.
If you are printing white words on a black or colored substrate, you will be using a spot color white. You cannot do both black and white ink at the same time so you will need a color change but you can use the same art. This is referring to screen printed transfers.
Digital transfers may do both on the same setup, but would require a digital printer that also does white inks. I don't know anything about the digital transfer process so someone else will need to clarify on how that is done if at all.