As of Release 4.0A, you no longer have to specify a unit of measurement for a quantitative inspection characteristic. This applies to both master inspection characteristics and inspection characteristics in a task list.
However, if you want the unit of measurement to remain a required field, you can set a parameter in the customer field selection (transaction SFAC) accordingly.
Parameters for the customer field selection
Master inspection characteristic: program SAPMQSDA, screen group QUAN
Inspection characteristic: program SAPLQPAA, screen group MK and MW