Subjective means depending on a subject, as opposed to objective which is independent of a subject. An objective measure would be the same, plus or minus some measurement error, regardless of who performs it or where it is performed.
Subjective does not mean arbitrary, nor is it necessarily less important than objective when making decisions or managing risks. On the contrary, judgments about the importance of organizational objectives come from top management whose judgment is the reason they are delegated the responsibility for making important decisions and enterprise risk management.
Nor does subjective necessarily mean inaccurate. However, it is possible and practical to obtain mathematically meaningful, accurate measures of subjectivity using pairwise comparisons as practiced in the Analytic Hierarchy Process. (See deriving ratio scale measures with pairwise comparisons for a demonstration of how this is done.)