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Consider you provide a LLQ class for traffic like VoIP. Further consider that most of your network doesn't analyze the traffic beyond looking at its DSCP marking. So, for example, VoIP phones mark their bearer traffic with DSCP EF.
Next consider, a server admin decides they want to prioritize their tape backups and so they too mark their backup traffic with DSCP EF. If you can see the potential problem, then you might see the need for trust boundaries.
Basically a trust boundary is where traffic on the "outside" isn't trusted while traffic on the "inside" is trusted. Traffic entering an edge port would often be outside your trust boundary, traffic between to network devices that only you control would often be trusted.
Were you might have different trust boundaries is where there different subsets of network devices controlled by different parties. Each might have their own trust boundary.
Other then not trusting edge ports or entry ports (e.g. traffic from the Internet), you'll likely only need one trust boundary.
