“Complex contracts” refers to contracts including: where the needs of the contracting authority cannot be met without adaptation of readily available solutions; contracts involving design or innovative solutions; where prior negotiation is required before a contract can be awarded due to particular circumstances related to the nature, the complexity or the legal or financial make-up of a contract or because of risks attaching to these circumstances; and where technical specifications cannot be determined with sufficient precision with reference to established technical standards, references or specifications.
read further here