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Each term in the TRT is assigned a unique notational code that conveys its location within the hierarchy. The first letter of the notation represents the facet or top term. Using the notational code it is easy to determine the terms relation in the hierarchy. The term's parent (Ae) will have one less letter (from the right end) as the term (Aea). Siblings (Aeg, Aes) will have the same notational code as the term except the last letter will be different. Children (Aeac, Aeah) will have one letter appended to the end of the notational code of the term.
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Facilities >
Transportation, hydraulic and utility facilities >
Air transportation facilities
Pmf:
Air transportation facilities
Pmfc: 
Pmfcbb: