Not everything in a mixture is always investigated as a part of a DOE. Often you want to add the same concentration or volume of a liquid to all of your DOE runs. These are constants - liquids that do not vary in their contribution to a mixture across all of the runs in your DOE design.
Some liquids in your mixture might always need to be present, but these are not factors in your DOE. Learn more about constants and when you might use them.
Learn how you can define constant mixture liquid components in Synthace.