If a category does not exist then you cannot compare it. Incomparable is when you have two disjointed sensitivity labels, that is a category in one of the labels is not in the other label. “Because neither label contains all the categories of the other, the labels can’t be compared. They’re said to be incomparable”
COMPARABILITY: The label:
TOP SECRET [VENUS ALPHA]
is “higher” than either of the labels:
SECRET [VENUS ALPHA] TOP SECRET [VENUS] But you can’t really say that the label:
TOP SECRET [VENUS] is higher than the label:
SECRET [ALPHA] Because neither label contains all the categories of the other, the labels can’t be compared. They’re said to be incomparable. In a mandatory access control system, you won’t be allowed access to a file whose label is incomparable to your clearance.
The Multilevel Security policy uses an ordering relationship between labels known as the dominance relationship. Intuitively, we think of a label that dominates another as being “higher” than the other. Similarly, we think of a label that is dominated by another as being “lower” than the other. The dominance relationship is used to determine permitted operations and information flows.
DOMINANCE The dominance relationship is determined by the ordering of the Sensitivity/Clearance component of the label and the intersection of the set of Compartments.
Sample Sensitivity/Clearance ordering are:
Top Secret > Secret > Confidential > Unclassified s3 >s2 > s1 >s0
Formally, for label one to dominate label 2 both of the following must be true:
The sensitivity/clearance of label one must be greater than or equal to the sensitivity/clearance of label two. The intersection of the compartments of label one and label two must equal the compartments of label two.
Additionally: Two labels are said to be equal if their sensitivity/clearance and set of compartments are exactly equal. Note that dominance includes equality. One label is said to strictly dominate the other if it dominates the other but is not equal to the other. Two labels are said to be incomparable if each label has at least one compartment that is not included in the other’s set of compartments.
The dominance relationship will produce a partial ordering over all possible MLS labels, resulting in what is known as the MLS Security Lattice.
The following answers are incorrect:
The number of classification in the two labels is different. Is incorrect because the categories are what is being compared, not the classifications.
Neither label contains all the classifications of the other. Is incorrect because the categories are what is being compared, not the classifications. the number of categories in the two labels is different. Is incorrect because it is possibe a category exists more than once in one sensitivity label and does exist in the other so they would be comparable.
Reference(s) used for this question:
OReilly -Computer Systems and Access Control (Chapter 3) http://www.oreilly.com/catalog/csb/chapter/ch03.html and http://rubix.com/cms/mls_dom