Notes:
t shall denote the amount of an explicit expression of trust of one node in another node. Then the local reputation R can be calculated by aggregating and combining the primitive trust values t.
In this example, B and C trust A directly. And B also trusts C. Thus, the reputation RAB that node A has when you ask node B is the aggregation of tBA and the result of the concatenation of tBC and tCA.
In contrast, A has a reputation of zero from D's perspective, because D does not trust in A, neither directly, nor indirectly.
In a centralised system, a large reputation matrix could be calculated which would comprise the reputation that every node has in every other node.