There are two different meanings for the question.
The first and the most common is "how clean is clean in relation to given parameters?" This refer to the verification of cleanliness, normally after a cleaning or manufacturing process when such a process has to meet a pre-determined level of cleanliness.
Another is "how clean is clean enough?" This refers to the degree of cleaning required to meet the operational requirements of the part or assembly in question and derives from the fact that, in the past, cleaning was done as well as possible using solvents without knowing if the degree of cleaning achieved was actually necessary.
The other meaning of the question is "how clean are the results of one cleaning process compared to another cleaning process?". This refers to the relative effectiveness of different cleaning processes.
All meanings are important and have direct bearing on the selection of alternatives. The latter, however, is required even if the answers to the other questions are determined.