A single transition is described in the <Processes><Radiative> table as a <RadiativeTransition> block. For example (truncated for clarity):
<RadiativeTransition id="PCDMS-R7452690" process="excitation">
<EnergyWavelength>
<Frequency methodRef="MCDMS-6648">
<SourceRef>BCDMS-344</SourceRef>
<Value units="MHz">104711.4035</Value>
<Accuracy>0.0057</Accuracy>
</Frequency>
<Frequency methodRef="MCDMS-6646">
<Value units="MHz">104711.3656</Value>
<Accuracy>0.0003</Accuracy>
</Frequency>
</EnergyWavelength>
<UpperStateRef>SCDMS-9240239</UpperStateRef>
<LowerStateRef>SCDMS-9240238</LowerStateRef>
<SpeciesRef>XCDMS-132</SpeciesRef>
<Probability>
<TransitionProbabilityA>
<Value units="1/cm">5.45485286998e-08</Value>
</TransitionProbabilityA>
</Probability>
</RadiativeTransition>
Note that the species (SpeciesRef), the lower level (LowerStateRef), and the upper level (UpperStateRef) are identified by their unique identifiers which backpoint to the given species in the <Molecules> table. The measured frequency is described in a <Frequency> block. There can be several frequencies for a single transition, measured by several sources.