For a Variable Font file to work correctly, it requires a minimum of two font files called masters, in which the glyphs need to be designed with:

• the same number of nodes and anchors;
• the same number of contours;
• in the same order and the same direction; so that interpolations or instances in between the masters can be calculated and displayed properly.

Masters can set up various types of axes (or design ranges) such as weight, optical size, slant angle, etc.
When there are multiple axes in a Variable Font, it is a Multiple Masters font.