A variable font file contains a minimum of one axis, which typically provides a fluid variation of styles that are visually congruent versions of a single design. Such a file can just contain width (wdth) and weight (wght) axes like many font families today, while making that range the basis for a “design space” of a font family with “instances,” rather than discrete styles.
In the replication of many existing font families, one variable-font file will contain the regular design and the original styles as named instances, allowing them to show in menus that match preexisting styles while coming from a variable font’s design space. Another variable-font file would contain the italic version with matching styles, if needed. [ex:1] It is also possible for a single variable font to contain regular and slant (slnt) axes, providing the family with an “italic” that is superior to computer-generated obliqued font styles. [ex:2] And, with an italic (ital) axis, a variable font can substitute contours from regular designs for an italic design.
The font specification also contains an axis that is new to desktop and web software: optical size (opsz). And variable fonts can contain numerous custom (or unregistered) axes, and thus styles in a design space new to users.