The ST_UNITS_OF_MEASURE table (available as of MySQL 8.0.14) provides information about acceptable units for the ST_Distance() function.
The ST_UNITS_OF_MEASURE table has these columns:
UNIT_NAME
The name of the unit.
UNIT_TYPE
The unit type (for example, LINEAR).
CONVERSION_FACTOR
A conversion factor used for internal calculations.
DESCRIPTION
A description of the unit.