Before S/4HANA Cloud release 1911, Units of Measure tab specify a custom conversion between a customer-specific unit of measure and a standard unit of measure for a customer material.
User must first select a unit of measure, and only then enter numeric value in the Customer Unit and Equivalent to Unit fields.
To make it more reasonable, the descriptions and field names changed as below after release 1911. And user must first select a sales unit, and only then enter numeric value in the Quantity in Sales Unit and Quantity in Base Unit fields.
Units of Measure -> Unit Conversion
Customer Unit -> Quantity in Sales Unit / Sales Unit
Equivalent to Unit -> Quantity in Base Unit / Base Unit