You can find a list of all subsets on the Google Fonts page for your font.
axes
Some variable fonts have extra axes that can be included. By default, only the font weight is included to keep the file size down. The possible values of axes depend on the specific font.
Used in next/font/google
Optional
Examples:
axes: ['slnt']: An array with value slnt for the Inter variable font which has slnt as additional axes as shown here. You can find the possible axes values for your font by using the filter on the Google variable fonts page and looking for axes other than wght
display
The font display with possible string values of 'auto', 'block', 'swap', 'fallback' or 'optional' with default value of 'swap'.
Used in next/font/google and next/font/local
Optional
Examples:
display: 'optional': A string assigned to the optional value
preload